Diana 27 – Part 7

by B.B. Pelletier

Part 1
Part 2
Part 3
Part 4
Part 5
Part 6

See? I didn’t make you wait very long. Today, I’ll reveal the bias that unfortunately crept into my test. While it renders my findings inaccurate, it doesn’t destroy everything. Though the velocities I have reported in this blog have all been potentially too low, they have also all been in the same region, relative to each other, and I think we can say that the new breech seals have worked. We just can’t say it as precisely as I had hoped.

A funny thing happened
And not the ha-ha kind of funny, either. It was the,”Oh no!” kind that humbles us all.

As I was finishing the last of the velocity chronograph work with the higher leather seal, I happened to notice that the left forearm screw seemed to be loose. It was loose and so was the other forearm screw and the forward triggerguard screw. In other words, all three screws that hold the rifle tight in the stock had gotten loose, so I tightened them all.

The next shot through the chronograph was 30 f.p.s. faster than the one before it.

OH, NO!

I felt exactly like the time on my psych finals when I discovered at the end of the test that I had been using a plain No. 2 lead pencil to mark the answer sheet instead of the special blue/black mark-sense pencil we had been given at the start of the test (those were the ’60s, don’t you know). Could I ever go back and cover every pencil-mark answer made in the three-hour exam with the right kind of pencil in the five minutes that remained? I could–as things turned out.

And so I could go through all the velocity testing with the raised leather breech seal AND THEN reinstall the raised synthetic breech seal and completely retest the gun! I could do all that, and I did, but I couldn’t go back and fix all the work that had gone before. Or, at least I am not anal enough to do it!

So, back through the tests I went and recorded new velocity averages with the now-tightened stock screws. I bet you want to know what happened. Right? Here you go. The BEFORE column is the average with the stock screws as they were. The AFTER column is the same test run with the stock screws tightened. And, yes, I did check the tightness of the stock screws as the test progressed, so the AFTER figures are all with tight screws.

SYNTHETIC RAISED SEAL

Before—>After

Eley Wasp
572—–>598

RWS Basic
629—–>658

Crosman Premier 7.9
579—–>605

RWS Superdome
564—–>586

RAISED LEATHER SEAL

Before—>After

Eley Wasp
587—–>602

RWS Basic
648—–>655

Crosman Premier 7.9
592—–>609

RWS Superdome
568—–>592

The gun works better (shoots faster) with tight stock screws. I’m sorry I didn’t check for it earlier, but I had no idea there would be such a huge difference! I knew stock screws affected accuracy, but velocity?

Hold me!
Then I thought about some tests I had conducted back in the days of The Airgun Letter. Some airgun tuners told me there would be a velocity difference between a rifle held loosely and the same gun held tight. I tested it and there actually was! You can test this for yourselves. First test the velocity of a spring rifle held as loosely as possible. Next, hold the rifle with a death grip and press it tight into your shoulder. You should see a significant reduction in velocity when the gun is held tightly. My Diana 27 drops about 25-30 f.p.s.

What to make of all of this?
The data from each of the tests indicates that the rifle is now shooting faster with any of the new breech seals. The seals of low or intermediate height are testing very well, and I have every reason to believe they would also be affected if tested with tight stock screws.

Yes, the results of all the tests are now suspect, because I don’t know the status of the stock screws for each of them. But the general trend toward better and faster velocity seems to hold throughout the series. I’m satisfied with the results of this exploration of breech seals and am ready to move on to look inside the Diana Model 27.

Before we leave this subject, however, let’s take a moment to reflect on what we’ve learned. From the start of this series to now, the velocity of this rifle has been increased by 200+ f.p.s., depending on which pellet you look at. All that came from simply fixing the breech seal, so it leaks less air. That’s a major boost in velocity, wouldn’t you say?

Finally, I think I got a diagnostic out of all of this: when the velocity of a pellet fluctuates wildly, look at the breech seal before anything else. The standard tests of feeling for a blast of air or talcum powder may not reveal what’s going on, so always suspect that seal first.

62 Responses to “Diana 27 – Part 7”

  • Mr B. Says:

    Morning B.B.,

    Loose stock screws–stumped us all? Who would have thought!!! Is that because some of the firing energy of the spring gets used moving the gun within the stock?

    Why does holding a gun with tight stock screw in a “death grip” reduce its velocity? It seems like the logic of the tight screws would also apply to the tight grip, ie, energy not being wasted by moving the gun. Please give me some help in understanding this one–thanks

  • B.B. Pelletier Says:

    Mr. B.,

    I haven’t the foggiest idea of why that works that way, but I expect before sundown that Herb, BG_Farmer and Rocket Jane will be writing formulae all over this blog, explaining it.

    It’s like the old one about the Thermos bottle. How do it know to keep the cold things cold and the hot things hot?

    B.B.

  • FRED Says:

    Mr. B has raised the same questions I had – I suspect that movement of the mechanism within the stock eatS up some energy from the spring but I have no idea why holding the rifle in a death grip would also influence the speed unless that more energy is absorbed by the shoulder as opposed to just letting the rifle recoil normally – but that intuitively doesn’t make sense. Looking forward to explanations and more formulae :).

    As for the thermos, it’s because it’s the smartest invention in the world.

    Fred

  • Mo Says:

    Mr. B,

    I dont know if this is the right example but imagine you’re in a swimming pool and push with your legs against a wall for that initial acceleration. Now assuming the wall moves when you push, won’t that affect your performance?

    I feel its the same principle here.

    Also, while holding the stock in a vice like grip, the action will have to work harder to recoil as opposed to letting the stock “float” in the palm of your hands, where movement is not restricted.

    I may be completely off track here. (been there before.. Lots of times..) lol.

    Mo.

  • Herb Says:

    RE: Loose screws – I have them too!

    Love this blog. Learn something all the time.

    The reason that the pellets lost power is because of the conservation of momentum. According to Newton, the pellet goes one way, the rifle the other. But it is momentum that is conserved, not kinetic energy.

    Mr*Vr = Mp*Vp

    The rifle and the pellet do split the total energy released as well, but not equally (unless the weights are equal).

    Energy(Total) = KE(rifle) + KE(pellet)

    If the mass of the rifle is much much greater than the mass of the pellet, then the kinetic energy of the rifle’s recoil is much much smaller than the kinetic energy of the pellet. KE goes as square of velocity.

    KE = 1/2 * m * V^2

    BB – Now I have to beg to differ. If you hold a rifle tightly, you are effectively adding your weight to the rifle’s weight to increase the mass of the “recoil system.” The system (you+rifle) weigh more than just the rifle. So the recoil saps even less energy, and the projectile goes faster.

    Herb

  • kevin Says:

    B.B.,

    Seems like you teach us something new everyday.

    Can’t help but think back to the time when you got out the yellow tablet in order to write down the short list of airgun topics you thought existed in order to start this blog. It isn’t a short list. Seems like the nuances of airgunning will never let you run out of topics.

    You have to be pretty happy with the velocity of your diana 27 now. What a great find.

    kevin

  • Vince Says:

    Curiouser and curiouser….

    I can see how an action that’s free to move would result in a lower velocity. After all, the gun will produce a certain muzzle velocity in relation to itself… if the gun is moving either forward or backward during the time the pellet is being accelerated the velocity relative to the chrony will change even if the velocity relative to the muzzle does not.

    What I don’t get is that the pellet is slower (relative to the chrony) when the stock screws are loose, while it is higher when the gun is held loosely. I would have thought that a loose stock and a loose hold would have been analogous events resulting in the same effect.

  • twotalon Says:

    B.B.
    I think I know the answer to why it shoots faster when held loose.

    During the rearward portion of recoil, the piston and the gun try to move in opposite directions. This is when the piston is in motion.

    When held loose, the gun is allowed to move to the rear while the piston is moving foreward….increasing the speed at which the piston and the front of the compression chamber come together. This causes a faster pressure build up.

    Then when the piston stops and forward recoil begins, the forward motion of the gun gives the pellet an extra boost.

    twotalon

  • Anonymous Says:

    Please excuse me but I have another question re: Weihruch HW 75 and Beeman P1.
    Can one use the HW 75 to eliminate squrrels at 50 feet or less? I chose the HW 75.
    Thanks,
    Earl Cox

  • Anonymous Says:

    BB – Airsoft question(s). I just recieved the S&W MP 40 airsoft (spring version) from Pyramid and I can't get it to shoot accurately at all. i'm shooting at about 20 ft indoors and can't even consistently keep my shots on a 6" x 6" pc of cardboard.. Are air soft bb's like pellets in the fact that you may have to find the right one to fit your gun? Is it unreasonable to think this gun would shoot a 2" group at 20 feet? Are there any accuracy tips you can provide for an airsoft pistol? Also, is it okay to dry fire these? Thanks for your help, I want to give this gun a fair chance, but if I can't get it to shoot more accurately, I may return it.

    -Aaron

  • Mr B. Says:

    Mo,

    Pushing off the wall when swimming is a great example. However, if the wall is flexable, ie, loose hold, I won’t be able to push off as hard because I’ve got to take the flex out of the wall first which is, in my mind, the same as a loose hold.

    A tight hold, again in my poor befuddled brain, would be analogous to a solid wall for me to push off, ie, more energy going into moving me forward.

    Herb,

    B.B. is making his statment based on imperical data so you can beg to differ with him, but that doesn’t change the data, ie, lower velocity when gun is held tightly. In this case your formula doesn’t seem to apply.

    Mr B.

    word verification is crimpled which is what this has done to my brain.

  • Vince Says:

    When I get a chance (!) I’ll try a velocity comparison test.

    1) Action out of the stock

    2) Action in the stock, loose screws

    3) Action in the stock, with the buttpad against a solid backstop.

    I think I’ll use a synthetic-stock Gamo, and maybe a Panther. Don’t wanna go tearing up any screw holes in a wood stock…

  • Anonymous Says:

    Good morning all!IMHO the secret lies in the energy reflected off the shoulder added to the foreward recoil caused by the piston suddenly stopping.the gun doesn’t travel far,but it does so at the exact fraction of a second the pellet is accelerating down the barrel!that causes it to be added to the thrust the pellet is already enjoying from the hot blast acting on it.to prove [or disprove]I would look to the Beeman P1 because the piston travels in the opposite direction…FrankB

  • B.B. Pelletier Says:

    HW75,

    The HW75 is the single-stroke pneumatic, isn’t it? It’s not the spring piston gun that’s thge equivalent of the Beeman P1. That would be the HW 45, no?

    No, the HW 75 is not good for killing squirrels at any range.

    B.B.

  • B.B. Pelletier Says:

    Aaron,

    Yes, airsoft guns are like pellet guns, in that they are sensitive to different types of ammo.

    No, it isn’t good to dry-fire a spring-piston gun of any kind.

    I hope you are shooting 0.12-gram BBs, because 0.20-gram are too heavy for that gun. I have heard that 0.16-gram BBs may sometimes work in spring-piston guns, so give them a try, too.

    B.B.

  • B.B. Pelletier Says:

    Vince,

    To be true to the test I did you’ll have to hold the forearm tight and push the butt back into your shoulder.

    B.B.

  • Herb Says:

    Did calculations…

    Shooting 14.3 grain CPs at 450 fps, KE of the pellet is 6.43 ft-lbs.

    6 pound rifle, velocity of recoil is 0.17 fps.

    KE of rifle recoil is 0.0027 ft-lbs.

    So if the recoil system was infinitely maasive, no energy would be lost to recoil and you add 0.0027 back to 6.43. But the difference is insignificant. You'd never be able to detect the increase in velocity.

    Dah….

    It occurred to me. There are three different recoils in the !@#$%^&* springer.

    (1) When the piston moves forward, rifle has to recoil back.

    (2) When the piston slams to a stop, the whole rifle will move in the direction of the piston motion (forward) to try to conserve the momentum of the piston. By holding the rifle tightly you are preventing this recoil. This causes the gas to compress more. More compression of the gas, and FASTER velocity.

    (3) When the pellet fires, the pellet goes forward and the rifle recoils back.

    I think the critical momentum transfer is #2.

    So I still think BB remembers the story backwards. I think holding the rifle tightly would increase the velocity of the pellet, not decrease it.

    Herb

  • CJr Says:

    Let me ask if this is contributing to the hold situation. I have read in this blog that before the pellet even begins it movement in the barrel the piston of a spring piston gun moves forward until it compresses the air to its max ability and then moves back causing a double recoil, one in each direction. Remeber this is before the pellet even starts to move down the barrel.

    This is the action that is significant enough to tear up good scopes not built for it. Therefore if this reverse recoil is so significant to affect a scope wouldn’t it also significantly, negatively, affect the force of the air pushing on the pellet?

    Holding the gun tight would cause the effects of the piston’s reverse recoil to steal pressure from the pressure created by the forward motion of the piston somehow.

    I’m not smart enough to explain how but that is my theory.

    -Chuck

  • Anonymous Says:

    Herb,with all respect to you,your recoil “measurement” is not accurate at all.you can move very fast for a brief duration.that is what the rifle does…I asked about this when you quantified recoil a few days ago. please explain your numbers for me…FrankB

  • CJr Says:

    Continuing thought:

    I think with the tighter hold, the reverse recoil of the piston is greater, because it’s not absorbed by the motion of the gun, and therefore causes a reduction in the total pressure sent to the pellet.

    -Chuck

  • CJr Says:

    Continuing again:

    It’s as if the reverse recoils sucks away some of the pressure built by the original forward motion.

    Ok, I’m done until further notice.

    -Chuck

  • Anonymous Says:

    BB – I was shooting .20 gram BB’s. I gotta say thumbs down to pyramind on that one, I picked the bb’s off of their reccomended ammo list for that gun. I’ll get some .12 and give those a try.

    Thanks,
    Aaron

  • Herb Says:

    FrankB

    RE: please explain your numbers for me…

    Recoil of rifle due to pellet?

    Mr*Vr = Mp*Vp

    Mr = weight of rifle = 6lbs = 42,000 grains
    Vr = velocity of rifle
    Mp = weight of pellet =14.3 grains
    Vp = 450 fps

    Vr = 14.3*450/42,000 = 0.17 fps

    KE(rifle) = 1/2* M * V^2

    OK, I fouled up here… I didn’t really use the KE formula shown…

    I used KE calculator at PA site. That works with WEIGHT in grains and FPS, not mass in slugs.

    KE = Mr * Vr^2/450400

    KE = 0.0027 ft-lbs.

    Weight is mass with its attraction by Earth’s gravity. So you weight less on the moon. Your mass on earth and your mass on the moon are equal. I was verbally sloppy in using weight and mass.

    M(slugs) = Weight(grains)/225200

    Now first KE equation would work…

    That make sense?

    If you don’t understand, please keep asking. If we disagree about something we may just have to agree to disagree. But I am more than happy to explain my reasoning. If you don’t follow something, there are other folks having the same problem.

    Herb

  • Herb Says:

    RE: Complex calculations

    Heard a phrase one that I loved when applied to skipping steps in a complex calculation.

    …And so it is intuitively obvious to the casual observer that x = …

  • FRED Says:

    One last action that’s not being taken into account in the calculations but it may have insignificant impact on forward and backward motion of the barrel and the forward motion of the pellet – the twisting or torquing of the rifle from the spring as it expands and uncoils. CJr. – congrats on beating me to the keyboard on bringing up the slight reverse or bounce of the piston as it compresses the air column until the resistance exceeds the energy left in the piston and spring. I had a lot of work to do this morning – for a change :)

  • Mr B. Says:

    B.B.,

    I hope I’ve got the right book, but in I Robot he put a covering over an ant hill to see how the ants would evolve if they weren’t killed every winter. The last time he checked they had built a forge and it looked like they were smelting iron.

    My point for that is, I had a mental pucture of you watching us and wondering how your topic for the day morphed into what it became. Just a thought.

    Mr B.

  • Herb Says:

    RE: DRECREASE in velocity

    OK, how about this?

    In physics there is a measurement for the coefficient of restitution. Basically take a ball and measure how high it rebounds compared to the height from which you drop it. The higher the ball bounces back, the more of its kinetic energy is returned to the rebound when the ball decompresses. A value of 1 would be perfect and the ball would bounce forever. Basically this would mean that compression/decompression doesn’t heat the ball at all. All the kinetic energy is sorted in the compression itself.

    When the piston slams into the chamber at the end of its stroke, it then rebounds. Since the rebound takes energy, this energy is lost to compression. By holding the rifle tightly you could be “hardening” the rebound. Thus the piston gets more energy on the rebound.

    This is really a sort of nasty dynamic problem. It would seem that if the rifle was absolutely immobile that you would get the greatest instantaneous pressure. This pressure would occur at the moment that piston’s motion stopped. But the highest instantaneous pressure would also result in the greatest rebound.

    However the gas in the cylinder can be modeled as an average pressure against the pellet. If the rifle is moving forward as the gas is compressing (a light grip) then the absolute pressure isn’t as high, so the piston rebounds less. But since the piston is compressing the gas over a finite amount of time the average pressure is in fact higher. The net result is that the “death grip” does in fact result in a velocity DECREASE compared to a “light grip.”

    Ought to know that BB is rarely goofs!

    Herb

  • Herb Says:

    Well again, I didn’t said hings very clearly. When the piston rebounds quickly, the gas decompresses quickly. By slowing the retreat of the piston, the pressure stays high longer.

    Herb

    Geeze – validation word is “dumsi” as in dumbsy – Go figure

  • kevin Says:

    Everyone,

    Re: The big picture about B.B.’s last two articles

    If you don’t already have one, please consider adding a small springer like the diana 27 or haenel model 1 to your arsenal. Very accurate, easy to cock, minimal noise and relatively low velocity and these little guns don’t need a scope since most have great, adjustable open sights.

    kevin

  • kevin Says:

    Herb,

    Your last two comments were very clear and what you said made sense to me……for the first time in a week. :-)

    kevin

  • tunnel engineer Says:

    One more for the thread….

    It would be impossible to qualitatively figure out the question of why the pellet moves faster when the stock screws are tight and slower when the rifle is held tightly.

    This is a very complex dynamic problem where the bodies are not rigid and the energy is transferred in waves traveling through the different parts of the action and reflected off or absorbed by other bodies in contact with it

    Also, the different parts of the mechanism are not really elastic bodies, they yield under the high stresses applied. So, holding the rifle tightly may cause more plastic deformation and thus transfer less energy to the pellet.

    The closest example that I can use based on what I do everyday is driving of piles (like those you see being driven for foundation of new bridges and overpasses). The steel hammer impacts the pile through a cushion. The cushion is often vital to be able to drive the pile as it allows about the same amount of energy to be transferred to the pile with each hammer blow but in a longer time (talking about thousandths of a second) and thus generating lower stresses in the pile. Without the cushion, the pile might yield (enter plastic state) with each blow and part of the energy would be spent in this plastic deformation and the pile would not advance as well. This is similar to saying that we are holding the hammer “looser” as we drive the pile. There is also a long list of other factors such as the impedance of the pile. Thus, in some cases the cushion helps and in some others it does not….

    In the end, the effect of the tight grip on this rifle will probably be different than the effect on other significantly different rifles.

    What happens when the rifle is fired clamped to a heavy table? I am sure many people have done this before

    Dirt Digger

  • Anonymous Says:

    Here’s another vote for tuning. The loose screws can really mess you up.

    My only guess about the unintuitive velocity change of the springer is that the loose hold allows return to point of aim which somehow maximizes the contribution of energy to the pellet which is otherwise routed off with the tight hold.

    Otherwise, I’m a little skeptical of mechanical models to explain this in terms of the working parts of the gun and the body considered as a solid mass. My experience with springers as the recoil propagates through the body is that the system involved is incredibly complex and outside our capacities to model.

    Matt61

  • Herb Says:

    RE: Piston rebound

    No model would be perfect of course. You’re just trying to account for the major factors. For instance I was able to show that the energy lost to the recoil of the rifle due to the pellet’s momentum was negligible, and could be ignored. Ignoring the factor makes the model “wrong” but that doesn’t mean that the model I proposed for the piston rebound is “useless.” It in fact gives you a hint where some tweaking might improve the overall power transfer.

    I have wondered before if a piston on a springer could be made like a dead blow hammer. That way the piston would rebound less, and the pressure would stay high just a bit longer at that critical part of the dynamic process of the rifle firing.

    Herb

  • Herb Says:

    RE: Piston rebound

    The other thing here is that the model is predicting that it is the retardation of the FORWARD recoil using the “death grip” that causes the velocity drop, not the BACKWARD recoil.

    Of course if we had a perfect mathematical model, we could just play with he numbers. We wouldn’t have to cut any metal to do additional experiments to see if the model was right.

    Herb

  • CJr Says:

    Herb,
    “…the model is predicting that it is the retardation of the FORWARD recoil using the “death grip…”

    That was my point. I was pointing out the backward recoil of the piston which implies the forward recoil of the gun.

    Have any of you other chrony owners tried this hard hold thing yet? And, has anyone tried it out on a pneumatic, CO2, or PCP to see if the results exist there? Or, another model springer for that matter?
    -Chuck

  • B.B. Pelletier Says:

    Mr. B.,

    Hush now and pay no attention to the covering.

    Just keep learning!

    B.B.

  • B.B. Pelletier Says:

    Everyone,

    1. I second Kevin’s suggestion about getting a small low-powered spring rifle like a Diana 27 or a Haenel Model I, II or III. And there is what I consider a great buy over on the Yellow Forum right now on a Diana Model 27, carrying the name Hy Score Model 807. It’s a .22 caliber that’s the twin of the one I own–or I would have bought it myself.

    Many of you may not be ready for something like this but a few are, so for you, here is the link:

    Sorry, it’s too late. The gun now has two different people interested in it. I will make that loss up to you soon with an airgun sale a thousand times larger and possibly better. Watch tomorrow’s blog for a teaser.

    2. Why has nobody tested what I said yet? You are all taking my word for it. No, I didn’t lie to you or trick you, but for gost sakes, try this for yourselves. Nobody made me any smarter than any of you, and, after reading the past week’s comments I see that I can’t even keep up with a lot of you.

    Test for yourselves! Maybe there is more here than we (I) know about!

    Back when Frank Mann wrote “The Bullet’s Flight…” he tested the accuracy of a rifle barrel attached to cylindrical rifle action by rotating it 90 degrees at a time while clamped to a one-ton concrete bench he called his “Shooting Gibralter.” Know what he discovered? That the accuracy of a barrel CHANGES with the orientation of the barrel around the axis of the bore! If that doesn’t start Herb nailing his 95 Theseus’s to the Wüttemberg Church door, I don’t know what will.

    B.B.

  • Herb Says:

    Chuck,

    I now agree with your statement at: March 11, 2009 10:51 AM. You nailed it first. Until I thought through the whole model, I just couldn’t connect the dots. I now understand what you were saying. Kudos to you.

    RE: And, has anyone tried it out on a pneumatic, CO2, or PCP to see if the results exist there?

    No they won’t. They don’t have a piston. By modeling I’ve shown that the recoil energy due to the pellet is trivial.

    Now the recoil energy as I calculated does ignore the momentum of the gas flowing down the barrel. But even if we multiplied the momentum to the target by 10, the recoil energy is still trivial. I’m sure that the model for any pressurized gas model isn’t that far out of whack.

    RE: Or, another model springer for that matter?

    Every model of springer, where the piston starts moving forward, should exhibit the behavior. The lighter the gun, the heavier the piston, the faster the light hold, then the more effect should be observed.

    Now we get to a point where I have to agree with tunnel_engineer. To write out the equations that completely describe this phenomena would be extremely hard. In other words, you should see the effect with other springers. But given specifications about piston weighs, spring compression strengths and so on, it would be nasty to derive an equation that would allow you to calculate that the difference is 37.5 fps for a Crosman G1, 23.7 fps for a B-7 and so on.

    Herb

  • CJr Says:

    Fred said:

    “CJr. – congrats on beating me to the keyboard…I had a lot of work to do this morning – for a change :)…”

    See, Fred…I was just covering your back.

    -Chuck

  • CJr Says:

    Yay! I just got an email from Pyramydair. They shipped my chrony. Now I’ll see if you guys and gals really know what you’re talking about.
    -Chuck

  • Herb Says:

    BB

    RE: “No, I didn’t lie to you or trick you, but for gosh sakes, try this for yourselves.”

    Never thought you’d LIE, I did think you could be mistaken in how you recalled something. Now I cannot explain why I thought it more likely that you’d be mistaken, than I’d be WRONG.

    ;-)

    Herb

    PS – Mann setup his shooting Gibraltar wrong.
    (1) center of mass of the whole system is below the boreline. Thus when rifle fires a torque is created.
    (2) Rifle barrel doesn’t flex the same in every direction. Some directions are stiffer to flex than others.
    (3) Lack of recoil to absorb some of the vibrations seems to have put other weird stress on rifle.

    1+2+3 = mess

    The gist in Mann was annoyed to discover that some marksmen could shoot better than his Gibraltar would. He built the behemoth to be the “ultimate” bench and it wasn’t.

  • BG_Farmer Says:

    BB,
    I never thought about loose screws affecting velocity, but they sure can mess up accuracy! My little Hammerli 490 shook off that tendency after about a tin of pellets, but I still check screws regularly.

    Regarding Mann’s experiment, the number of factors affecting accuracy is astounding — the real wonder is that there are any consistent, accurate rifles! I don’t want to seem mystic, but I think really good shots must subconsciously integrate all those factors and correct for them. It would be interesting to give a really top-notch shooter a bench-tested doppelganger of his rifle and see how he shot it — I would bet money he couldn’t shoot it as well until he got time with the clone.

    No formulae from me today: Analyzing the recoil on a springer gives me a headache:).

    PS. verfication = redna– pretty close to the truth for me (rednack):).

  • FRED Says:

    Seems to me there are a lot of loose screws around, starting with myself. I’m going to blame my lack of extreme accuracy on some loose screws. It’s the American Way – can’t be my fault, must be someone else’s. :)

  • BG_Farmer Says:

    Fred,
    In my case, its the “trigger nut”:).

  • Anonymous Says:

    “I will make that loss up to you soon with an airgun sale a thousand times larger and possibly better. Watch tomorrow’s blog for a teaser.”

    Could it be free guns for all of us faithful blog readers?? I hope so.

    -Aaron

  • Anonymous Says:

    Herb,I was away from the comp. all day,anxious to return.I’m still not satisfied with important facts.if we could back up to the facts…the recoil isn’t a simple function of pellet pushing back.that energy alone would be very small…I’m referencing the whole moving mass consisting of spring,piston and seal.they lurch foreward driving the rifle back.a tight hold keeps THAT energy from rebounding the gun foreward.add to that the sudden stop {and/or rebound of the piston}which would be suppressed by a tight hold.both those energies combined happen exactly when the pellet is recieving foreward thrust from the air charge.this accounts[albeit without formulae]for the difference in velocity between holds.IMHO,of course.Herb,please read this twice before you respond.Then I would love to hear back! P.S.does anyone have a P-1 and a chrony?? FrankB

  • DB Says:

    Vince,
    TF97 went on the UPS truck Monday. Well it went to the store Monday. According to the tracking it is still less than half way to your house.

    Sent you an email with the tracking number. Hope you got it.

    DB

  • DB Says:

    All,
    Dang.. I never get to read the blog until late. And you guys already had the puzzle figured out.

    Makes sense after you think about what the heavy piston does to the rifle as a whole. The mass of the little pellet is nothing compared to that. If fact that is why that little pellet flys out so much faster than the piston.

    Good one,
    DB

  • Anonymous Says:

    Herb,

    This isn’t to disparage attempts to understand, and I do think that the contribution of the forward movement of the piston is illuminating. But it is a question of what a model can be expected to do.

    The model considers the whole shooting system in terms of the piston, the pellet, and the rifle and perhaps the whole body. But what exactly goes in with the artillery hold? We know that it is as loose as can be. But it is not infinitely loose like B.B.’s idealization of a gun suspended by a string. If that were the case our whole bodies would collapse. As far as I can tell from shooting (and correlating with BG_Farmer’s comment), the subconscious somehow learns to let the recoil move through the body while dampening it to some degree to enable a follow through. This brings into play the astoundingly complex structure of the body with the elastic tension of all the materials of bone, muscle, sinew that make it up. In view of all this, to suppose that tiny differences between parts of the rifle will explain the velocity effect doesn’t seem credible to me. If the model corresponds to observation, it may be right or it may not, but unless it can explain all of these other factors or give a convincing reason why they aren’t relevant, we won’t know for sure.

    Along those lines, it seems to me in considering shooting as made up of material technology and shooting technique, there is a disproportionate amount of time spent on the technology. As an example, I finished reading a book about amateur gunsmithing; real wishhful thinking here. The author could do all sorts of incredible things including jeweling bolts and actions, glass-bedding, dremeling screws, recrowning muzzles, stoning triggers, and building whole custom rifles and many more things that I cannot remember. But the accuracy reported for the finished products wasn’t all that great. It seems he would have profited from putting more rounds downrange.

    On the lines of shooting technique, I’ll share a (for me) startling discovery. I had noticed that usually I could trace a bad shot to ignoring some shooting fundamental as I concentrated on holding the sights on target. Then, it occurred to me to wonder what would happen if I worried less about keeping the sights on target (once they’re on) than on making sure to hit the fundamentals in sequence 100% of the time. Sure, I would throw a few, but I wondered if there would be a net gain relative to the many screw-ups from working the sights onto the target. Well, lo and behold with a posture of relative indifference to the target while focusing my attention on my version of the shooting fundamentals done in a strict time sequence, the accuracy improved a lot–not just in the aggregate but for each shot. Skill was converted more into being diligent and consistent. In a way it was kind of sad–Matt wasn’t much of a shot after all since any extra effort he brought to bear seemed to make things worse. But on the bright side, I’m still the one pulling the trigger. :)

    Anyway, if you’re getting bored, you might try a drill of just making sure all the fundamentals of breath holding, trigger press and follow-through are there in sequence without worrying about where the pellet lands, and you might be pleasantly surprised.

    Matt61

  • Herb Says:

    FrankB

    RE: Recap

    There is a real good demo of a springer action at:

    http://www.arld1.com/rifledynamicssmaller.html

    (1) Loose screws

    The loose screws don't cause a problem with rifle "recoil." It is actually lifting the mechanism. So the torque isn't rotating gun about its center of mass, it is lifting the metal parts. A six pound gun 2 inches is one foot pound.

    Using BB's measurements on CPs, we're looking for:

    7.9*605 – 7.9*579 = 0.54 ft-lbs

    So this is in the right ball park.

    (2) Diana 27 & death grip, Diana LOOSES power

    This is the really interesting case. There are a number of recoils here

    (a) Piston/spring accelerate forward which results in a rearward motion of the rifle butt.

    Now if rearward motion were prevented with death grip, less energy would be lost to rifle system's (you+rifle) recoil. This would make the piston/spring to move faster forward.

    Faster piston, more air pressure & FASTER pellet. Since overall pellet slows, this can't be the controlling consideration.

    (b) piston reaching end of cylinder and rebounding

    Here there are two factors

    (b.1) Piston slams into end of cyclinder compressing air and stops. Rifle goes forward to conserve momentum. If you stop this recoil, then the pressure of the gas is higher and the pellet goes faster again. So the energy lost due to the forward recoil of the rifle itself is the wrong way.

    (b.2) Now take a longer view of air pressure in the chamber besides just instant that piston stops. Imagine pressure over time as piston accelerates, stops, and reverses direction. We'll assume that the pellet fires before the second rebound of the piston.

    Now what we are trying to create is not the highest instantaneous pressure, but the highest average pressure from the time the pellet moves till it leaves the end of the barrel.

    As the piston recoils, then the pressure in the piston chamber starts dropping rapidly because the volume in the piston chamber is increasing not decreasing. So how fast the piston recoils is critical.

    If you have a death grip on the rifle, then the piston creates the highest instantaneous pressure which causes the piston to rebound with the greatest velocity. A "softer" hold lets some of the momentum be transfer to the rifle which lessens the rebound velocity of the piston. Less velocity of piston retreating, higher average pressure even though the peak pressure will be lower.

    (c) pellet leaving barrel

    When the pellet leaves the barrel, then it has forward momentum. The rifle system has to have backward momentum to conserve momentum. he pellet loses the energy to recoil of rifle. If you stop this recoil, then the pellet moves faster.

    So the only factor which would cause the Diana to lose velocity with a tighter hold is that the piston rebounds faster.

    Does this help?

    —–

    Matt,

    I agree, I spend too much time thinking and not enough shooting. Having a mental routine is key to success in any sport. Certainly true in golf as well for example.

    It is like old joke, first golfer asks second "Do you inhale or exhale with your backswing?" Now with second golfer trying to hit ball while figuring out answer to question, he plops the ball into the lake.

    Back to shooting. The torque of a springer lifts the barrel. The barrel is lifting as the pellet exits. You want pellet to exit same point each time. If you try to tightly hold the foregrip, then all sorts of nasty muscle twitches occur that destroy accuracy. Think of your forward hand as just a platform from which the rifle will lift.

    Remember all discussion about holding baby in a car crash? You can't. The deceleration occurs so quickly that the baby weighs a ton, not 10 pounds. Same thing is happening with springer recoil. the !@#$%^&* rifle is just going to climb and you can't stop it.

    A different notion would be swinging a rock on a string. When you release it the rock goes straight. In order to hit the target you have to release the rock in exactly the same spot every time.

    So guys, does this help?

    Herb

  • Vince Says:

    All this talk lately about wonderful, lightweight, easy-to-shoot springers made me wanna go out an buy – the heaviest and crudest springer I know of.

    I have no idea why.

    But the BAM B22 showed up today. Granted, it was only $115 – but still – why? Why? WHY???

    It arrived with a VERY pretty (for BAM) reddish stock – with a very large and nasty crack in it! And not from shipping. Velocity is about 80fps under what it should be.

    Maybe I wanted something as heavy as my Garrand that wasn’t made nearly as nicely as my 48. For some odd reason which I cannot fathom.

    Oh, well… live and learn.

    Who am I kidding? ME LEARN???

    (sigh)

  • Anonymous Says:

    OK folks here are some thoughts to ponder
    I don’t know any formula’s but all this can be proven or
    disproven with testing.1st I’ll wager that with the same power plant
    a heavier rifle will have more velocity than a lighter rifle.
    At least 2lbs.lighter to have a noticeable fps.difference.
    The reason being,the extra weight will help control detrimental vibration
    and movement.up to the point of diminishing returns,similar to
    barrel length in co2 guns.BUT a solid mount or “deathgrip”on either gun
    will have slower velocity because it hampers the natural movement of the design.
    2nd I’ll also bet that if the experiment were tried with one of the
    few guns where the piston compresses air to the rearward of the gun,then we’ll
    get opposite results.I.E that the loose hold will produce slower velocities.
    The reason I believe this has to do with direction of recoil.
    All these comments refer to springers.
    A loose hold on a powder burner will result in slower fps.although the difference
    would be slight,and harder to detect.simply because the bullet is moving a lot sooner in the
    recoil cycle of a powder gun
    I’m hunkered down and waiting for the incoming bombardment FIRE AWAY.

    JTinAL

  • kevin Says:

    Vince,

    I admire you.

    You seem to seek out challenges while other people, like me, avoid them.

    I seem to sense another multi-series article because I have faith that you can turn that frog that is a heavy gun, with a nasty crack in the stock into a prince that is a great shooting economical gun.

    If anyone can turn a bam b22 into something special it’s you.

    kevin

  • Anonymous Says:

    Herb,

    Actually, I’m working on my rock slinging but have not put in enough time to see results.

    As for the springers, one thing I really like about them is the way they jump when you let off the shot but when the sights settle, you see (if everything was done right) the pellet right on target. Miraculous. To do this, though, is more complicated than simply going limp. How is one to tell the exact right tension to maintain to deliver each shot the same? For me, it seems mostly to do with a follow through that involves keeping the eyes on the target. I know this would seem to violate the wisdom of keeping your eyes glued to the front sight. But for me, so far, switching from front sight to target at the discharge seems to provide just the right guidance for the muzzle. It’s as though the shooting process depends on pushing things to the subconscious or some other part of the brain that is much better at coordination and judgment of distance than the higher faculties.

    Vince, how wonderful to hear that you are part of the BAM crowd. I’ll be eager to hear your impressions. My own experience is that these are super rifles with great barrels WITH a tune-up. For me, I just sent it to Rich from Mich, but you can do it all yourself. Are you sure that the crack is not from shipping? That’s what happened to my rifle, and PA says that they see this all the time.

    Matt61

  • Vince Says:

    Matt61, neither the packaging nor the gun show any signs of impact damage, so I don’t think it happened during shipping. I emailed Xisico’s sales, info, and service departments as well as bestairguns and sent them pictures of the damage. I requested that they just send me a new stock, we’ll see what they say. In any event I’m NOT spending $20 to ship the gun back to them. If they refuse to send a stock I will attempt to repair this one.

    As for the velocity I believe I can probably take care of that. The cocking pressure at the first ratchet catch point is 35lbs+, so it’s a fair assumption that the spring is OK. Which means there’s a sealing problem, and either the breech or main seal ought to be easy enough to fix.

    I still don’t know why I bought this thing, I guess just because it’s a brute. I’m wondering of the B21 might be earmarked for discontinuation, as it’s selling pretty cheap. I’m not gonna try and make it into a Diana 48, because I’ve already got one of those. I’m gonna make this one as good as it can be, but I’m not looking to make it into something it’s not. I guess I will keep it simply as a tribute to the Neanderthal in all of us…

  • Anonymous Says:

    To go off topic I was wondering is the daisy 901 rifle supposed to be made after a gun used now in the mid east because it looks like a modern war gun

  • kevin Says:

    Anonymous with the Daisy Model 901,

    I’m not aware of any firearm that the 901 is modeled after. The Daisy Model 901 does look very similar to the Daisy Model 880 however.

    kevin

  • Anonymous Says:

    Hi BB,

    Your series on the Diana 27 is timely as I just bought a really nice old HyScore 807. My only complaint is the rifle buzzes like a can of angry hornets when fired. I was thinking a bit of tar on the spring and maybe some heat shrink on the guide. Were you going to show the break down of the 27 at some point? I just don’t want to dive in because I really like this vintage rifle and I don’t want to screw it up. I would rather send it to someone who know what they are doing if it is going to be tricky.

    Thanks, Allen

  • Vince Says:

    Allen, I wouldn’t go for heat-shrinking on the guide. But some tar on the spring will definitely help. How’s the velocity on it?

    The 27 isn’t too bad to take apart, although the 3-ball-bearing trigger can be a bit tricky. I’ve done a couple of 27′s and a 25 (basically a shorter version of the same gun). If you don’t want to do it yourself, perhaps I can help. If you want, email me at vfblovesnancy@yahoo.com

  • B.B. Pelletier Says:

    Allen,

    Yes, I am going inside my model 27. As vince says, they aren’t too difficult to disassemble, but putting them back together requires a trick, which I will be showing. For you, however, I will tell it right now.

    The secret is to use sticky grease to hold the trigger parts together as an assembly until you get them back inside the gun.

    B.B.

  • Anonymous Says:

    Thanks guys, I am looking forward to further parts of this series. My back yard is SO small (I can barely fit in a 25 yd range) the little 807 has been a good fit.

  • GunDa Bacha Says:

    Hello there..

    its very easy to understand that holding airgun loos always improves its efficiency. but the question is how ?

    its not the pressure or anything else but the time (how fast the piston hits its dead End)

    example: distance between cocked piston and dead end is 15cm

    *.. if we hold stock very tight .. and fire the gun .. piston has to travel 15cm (One way) and it takes 1 second.

    *.. if we hold stock very loose … and fire the gun … piston has to travel 7.5cm(will take 1/2 second) the rest of 7.5cm gun moves backward which indirectly reduces the distance in between piston and Dead End. (plz remember.. when gun moves backward doesn't mean it is taking piston backward also. at that time piston is going forward with double speed.

    that's why ..artillery hold always helps to improve airguns efficiency.

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