Shimming a Diana breech seal

Introduction by B.B. Pelletier

Guest blogger
Here’s another guest blogger. Vince Brandolini shows us how he shims a Diana breech seal to gain more energy. Vince has instructed several readers on this blog how to do this and they report similar velocity gains.

If you have an airgun story to tell, maybe you can tell it here. I’m looking for some budding bloggers who would like to help me write this blog. If you would like to write a post for this blog, please email me at blogger@pyramydair.com.

Bloggers must be proficient in the simple html that Blogger software uses, know how to take clear photos and size them for the internet (if their post requires them) and they must use proper English. We will edit each submission, but we won’t work on any submission that contains gross misspellings and/or grammatical errors.

Okay, Vince – take it away!

Shimming the Diana breech seal

By Vince Brandolini

I had gotten my first Diana rifle about 15 months ago – a wood-stocked RWS 34. I found it to be a bit of a disappointment in several respects and ended up selling it after a few months. One reason was the velocity; it generated under 14 ft-lbs at the muzzle. That’s no big deal, but the rifle certainly took more effort to cock than, say, my Gamo Shadow or Crosman Quest, either of which would handily out-power it.

Giving the 34 one more chance
Still, I did sort of miss the rifle. After hearing a lot of good things about the Panther variant of the ’34 (including BB’s review), I decided to take a crack at one. The example I got was a factory-refurbished model, and, frankly, all the reviews were right on the money. It was a much nicer gun to shoot overall, although the velocity was still low.

It didn’t worry me too much until I compiled a list of the mechanical specs of all my springers – including the potential energy stored in the powerplant when the gun is cocked. I then bounced that number against actual muzzle energy and found that all my guns came out at over 30% efficient – except for the Panther which came in at less than 28%. That didn’t sound right to me.

Huge increases in efficiency & velocity!
My suspicions were confirmed when I obtained a .22 caliber Panther a short time later, and it was lobbing Crosman Wadcutters under 600 fps (under 25% efficient). After some investigation, I found that shimming the breech seal resulted in a velocity over 700 fps. That made me wonder about my .177 Panther. So, I shimmed THAT breech seal and the velocity (10-shot average, 7.9 Crosman Premiers) went from 868 to 938 fps – a 17% increase in power and a new efficiency of 32.5%. Incidentally, the efficiency of the .22 is now close to 37%.

Dianas aren’t the only guns that could use a shimmed breech seal
When I found a similar problem with my very recently acquired Ruger Air Hawk (which is a BAM-built semi-clone of the ’34), I began to wonder if this might be a common issue.

How it looks on the inside
Below is what the typical Diana-type breech and seal look like (the pictures are from the near-identical Ruger Air Hawk).


The breech reveals a simple o-ring pressed into a groove around the barrel opening. If you remove the o-ring, you may or may not find a factory shim under the ring. Both my Panthers had shims, while the Air Hawk did not.


When you remove the o-ring, you may find a factory shim under the ring. Both my Panthers had shims, while the Air Hawk did not.


This is a side shot of the Air Hawk breech. There’s little protrusion from the breech face. That can lead to velocity loss!

Making & installing the shim is just about a no-brainer
This isn’t an expensive or time-consuming process and no mechanical knowledge is required, yet I found it was one of the quickest ways to immediately get a velocity increase. Let’s get to it!

The seal groove on the Ruger Air Hawk has a .540″ outside diameter and a .340″ inside diameter (measurements are approximate).

The tools…


I was able to make usable shims with 3/8″ and 1/2″ hole punches. I had a cheap one with interchangeable heads.

The shim can be made out of a variety of materials. Diana uses steel, while Crosman (with a similar setup) uses a thick paper. I settled on plastic from a coffee can lid, which yields a pliable shim about .015″ thick, and aluminum from a soda can, which is about .005″ thick.

Making the shim is a simple 2-step process…


First, I used the larger punch to make the outside diameter.


Second, I used the smaller punch to make the inside diameter.

The same procedure is used regardless of the material. If you use aluminum, the punching process will probably distort the ring, so make sure you hammer it flat before installing it.


The tricky part is that second punch. If the tool isn’t centered properly you get an uneven shim that looks something like this. If it isn’t too lopsided, the shim can probably still be used (as is the case with this one), but it’s best to have the second punch centered as much as possible.

What if you need to replace an o-ring?
If you discovered that your gun’s o-ring is torn, gouged, ripped or in some way damaged because that’s how it came in the gun or because you did something when removing from your gun, you can buy a replacement. I checked into Umarex’s replacement breech seals, and found that they charge $4-5 for one. If you need only one, that might be the way you want to go. On the other hand, I was able to buy a bag of 100 McMaster-Carr o-rings for just $3.


Clockwise from upper left: yellow plastic shim made from a coffee can lid, aluminum shim made from a soda can, McMaster-Carr o-ring, Umarex breech seal.

Installing the shim


Once the shim is punched out, it fits easily into the breech groove…


…and the seal protrudes far more prominently.

A few minutes of work increases velocity


In this case, I used a single coffee-can shim (.015″) to increase velocity from 852 to 871.

Yes, it’s still slower than my Panther, but the Air Hawk isn’t a perfect clone. The powerplant has a shorter stroke (by about 15mm) and a slightly softer spring. Out of the three guns I shimmed, this one improved the least. But, the improvement is unmistakable (it was observed over a series of several shots), and it brought the powerplant efficiency up to a reasonable 32%.

Later, I found a fiber washer from McMaster that seems to work well as a breech seal shim. It’s part #90089A330 and sells for $10 for a bag of 100. It’s about .016″ thick, so it won’t do if you’re trying to shim a very small amount.

This naturally leads to the the question – how do you know if your rifle needs it, and how much should you shim? There’s no set rule. My Diana 350 didn’t need any shims, and frankly I don’t know of any way of telling beyond trial and error. There’s a “wax paper test” for a leaking breech seal: load the gun and put a strip of wax paper between the breech and the compression tube as you close the action. Fire the gun. If the seal leaks, it’s supposed to tear the paper. Perhaps this works in some cases, but it didn’t work for me.

That’s about it. I suspect that my first 34 (the one I sold) suffered from this problem. Plus, Hank wrote that his .22 Panther gun picked up 100 fps.

57 thoughts on “Shimming a Diana breech seal

  1. Matt61,

    Daystate is coming out with a new airwolf in june. You may want to wait for it! Just go on Daystates wed site to see it. I would recommend the theoben OVER the airwolf due to the simplicity. I have one theoben thats going strong after eight years of hard use.


  2. Great Blog by Mr Brandolini, thanks for posting it BB. Since sending the Sierra back to Crosman for repairs, I’ve spent alot of time getting to know my Marksman/Norica better. I’ve been very pleased with it, and it seems to improve the more I use it. As usual, I was shooting with Destroyers, and decided to try out some Cromags. They fit snuggly in the breech, and fired fairly accurate. I examined the breech seal, and it does look like time has taken it’s toll. I was going to remove it and see if I could soak it in something like silicone etc. to bring it back to life, but after reading how to shim the seal, I’m going to give it a ‘shot’! I have punches in both standard and metric. Hope this will improve the velocity of mine as well. But I’m sticking with the Destroyers. Thomas


  3. Thanks Vince. What a great idea! Very well written and illustrated. Gotta get the punch and do mine. An almost free increase in performance. Thanks



  4. Along almost the same line as what tom said…how do you remove the breech seal without damage to the surrounding metal. What tools are you guys using to dig that seal out? It’s not glued on is it?

    Thanks
    Jim


  5. Vince -

    Can you share the details of the methods and math you used to calculate spring kinetic energy and overall efficiency?

    Thanks!

    Phil



  6. Actually, at least one breech seal I came across looked to be glued in – a Daisy Powerline 1000.

    As for getting them out, I use an O-ring pic or a small jeweler’s screwdriver. If the surrounding metal gets nicked a tiny bit it’s generally no big deal.



  7. Good morning Vince.
    This may be a bit off topic.

    I have a Beeman RS2 with 2 barrels. The .177 shoots very near 900 fps with cpl, but the .22 shoots at 600 with cp and 500 with Kodiaks. The .177 produces a great deal mote M.E. than the .22.

    Is It common for a two barrel gun to be so much better in .177 than a .22 for M.E.?

    By the way, straight out of the box there was no detonation and very little smoke.

    The rifle has a fairly smooth and modest firing stroke, but sure does cock hard.

    twotalon


  8. Twotalon, I’m not familiar with how the dual-caliber Beeman works, but those numbers are off. 900 with CPL should be (at least) near 700 with 14.3gr CP’s.

    I believe the RS2 has the basic powerplant of the AR1000, if it does even the 900fps sounds a little low. My Walther Force 1000 does about 970 with CPL’s – I’m wondering if you have a breech seal problem as well.

    Vince


  9. Thanks Vince.

    Will shim up the O-ring a bit when I get the chance.

    Should have posted the K.E. in both cals, but the MV was easier to remember.

    Interesting note……
    Both barrels are bent downward to correct for “reverse droop”. Right out of the box that way. Barrels would have drooped up otherwise. Trigger pull is not too bad, either. The Chinese seem to be trying to get something fixed.
    Now they need to learn how to make a good barrel.

    twotalon


  10. Phil, there is a difference between spring kinetic energy and potential energy. When the gun is cocked and the spring is in tension we are talking about potential energy.

    The energy is basically the area under the “pressure vs. compression” curve between cocked and uncocked points.

    Fortunately since these are constant-rate springs, this is a simple matter. Take the average of the spring’s preload and cocked pressures and multiply by the cocking stroke to get the potential energy.

    It can be impractical to measure the actual pressures (depending on the equipment you have available), so I calculate everything based on spring rate which (in turn) I calculate from the mechanical specs of the spring.

    BB, I can go into more detail on this – but is there any chance this subject might be best reated in another guest blog?

    Vince


  11. Vince,

    So define “shim” for me. Ha ha. Just kidding. I believe I have figured it out by now. Nicely done. This sounds like it’s actually doable. Maybe after I read up on spring gun tuning and learn to disassemble a rifle.

    Odd that the RWS 34 needs this kind of work. I sounded like a very high quality gun.

    Matt61


  12. Vince,
    Nicely done blog.
    My local Tractor Supply and Lowes(in the plumbing faucet repair section) have a decent O-ring selection, too.

    I am interested in your calcs because the “science project” part of the hobby is half the fun for me. Engineer’s syndrone. You too? LOL! I’ve used the calculators on various spring manufacturer’s websites and gotten good correlation with empirical measurements, but haven’t take the additonal step of then calculating effeciency from actual velocity on a springer. That would be interesting. Also, any ideas on where the other 70% goes? Cardew had some good information. Engraving the pellet, heat, pushing the air column, and as you’ve so clearly shown, leakage! Other losses?
    Well done!
    Pestbgone


  13. B.B.

    How about this as a blog topic or at least a source of anecdotes: airgun obsessions? I remember you mentioned one guy you hosted for a week or so and after introducing him to match pistol shooting that’s all he did all day long.

    I’m getting a little worried. The other night two hours of shooting passed in a flash although it seemed like I was just getting started with all of my guns. During meetings, I sometimes dream about the crosshairs dancing over the target before the final drop…

    And for those of you worried about missing shooting time because of work, here is a drill. You pick out letters on your computer screen with closed loops like p’s, o’s, and even c’s, drop the point of the cursor right into the middle and left click. You can be zapping those letters all day long.

    Matt61




  14. Vince -

    Thanks for the potential vs. kinetic energy clarification.

    B.B. -

    I’ve love to see a blog entry on calculations like Vince has done.

    Pestbgone -

    Like you, I’m curious about the physics of what’s really going on inside an airgun. It would seem similar energy is available for a springer and a single-stroke pneumatic (if they both require similar cocking force). Yet we know they typically result in different performance (i.e., why aren’t there any 1000+ fps single-stroke pneumatics?).

    A realistic, repeatably-measurable understanding of the force – and losses – in the system can help us understand how to get the most out of an airgun.

    Finally, B.B. -

    Thanks for your time an effort in taking care of a remarkably informative and useful resource!

    Phil L.


  15. Vince,

    Excellent job on the blog. Ingenius idea using a plactic coffee can lid. I had always thought to use shim stock, but your method is far, far easier to punch and size.

    Nick and I have been emailing back and forth about punching leather breech seals for the Predom pistols. So your punch technique works great for that, too.

    The Remington (Crosman) Summit I got a month ago was leaking terribly. I was able to size up to a much thicker o-ring. Couldn’t get the actual diameter I needed though, so I cut a small section out of it. The pressure alone from the fit in the channel keeps it from leaking at the seam. It’s a really tight fit and it now seals perfectly.

    I use a dental pick to remove small o-rings. Very handy tool for small jobs.

    Derrick


  16. Matt61,

    Reading between the lines again, sorry about that…

    I think that when I say target bull, you’re thinking I mean the very center of the 10-ring.(?) Bull means the ENTIRE black disc part on the target. So, on a 10 meter pistol target, that means the 7-ring and inward.

    Ideally, your match pistol would have an adjustable or a replaceable FRONT sight that would allow adjustment so that it appears to your aiming eye as the exact same width of the “bull”. This makes it easier for your eye to align that black bull right on top of the front sight. Good target pistols also have adjustable notch widths on the REAR sight to then compensate for the changed width of the front sight so you get some daylight on either side. Did that make sense? Man, I hope so.

    Maybe BB can show this relationship in his upcoming blog.

    You mentioned that this was an unconventional aiming technique. It’s actually the norm in precision target shooting.

    The ISSF videos are addicting, aren’t they?

    Derrick


  17. Hi there Vince…I do this breech seal shimming with any new airgun I get, and it has *always* led to an increase in performance.

    One pistol in particular, the Beeman P17…shimming allowed me to get up into the 430fps range and at least 420fps consistently. Those who haven’t broken the 400fps barrier with their P17, shimming will do it!!!
    –Dave Ennis


  18. Thanks again Vince.

    I took a break from other things and made a shim out of construction paper for the Beeman.

    I checked the end of the barrel where it came through the front breech part and found it to protrude a bit. (.177 barrel) Filed it off flush and got lower 800′s rather than nearly 900 fps as was original.
    Installed the shim under the O-ring and got lower 900′s with cpl..920-940 range.

    Tried the .22 barrel and got a jump from previous 600 fps to lower 700′s.. 720-730 fps.

    Velocity a bit jumpy in .177. may need to change pellet weight.
    Velocity pretty steady in .22.

    twotalon


  19. bb-
    will the hold you use on an air rifle effect its zero? For instance you zero it on a bench will shooting offhand give a dicferent zero?


  20. Twotalon, sounds like you picked up noticeably in both calibers. You might want to try shimming it a little more… I thought I had my .177 Panther dialed in when it went from 860′s to 930′s… on a hunch I shimmed it a few thousandths more and it is now averaging over 950 (all with CPL’s). If I shim it any more the bottom part of the seal gets abraided by the transfer port (as Gamo’s are known to do), so I think I’m maxed out.

    Funny how it went from one of my weakest .177′s to one of my strongest…

    Vince




  21. Vince……
    I can feel the breech closing on the O-ring now. Don’t think I want to shim it any more. May smear a bit of silicone on it.

    This is a new rifle…less than 100 shots total through it.

    Had to fix a .22 barrel problem…
    The last 1/4 inch at the muzzle end had bad machining marks in the bore. Lopped it off to a tight clean spot just behind that and re-crowned.

    Have not shot outdoors with it yet.

    Might just leave it as is until I give it a good beating with junk pellets.

    twotalon



  22. Vince,

    What about doing a very shallow countersink on that Gamo’s transfer port opening to prevent the abraded breech seal?
    I’m thinking just a sharp drill bit twisted by hand here.

    Dave,

    Thanks for the hint on the P17. Never occurred to me to try on that gun. What did you shim your P17 out with? I’m doing that tonight.

    Derrick


  23. Hi Derrick, as B.B. pointed out last night, using a 6 o clock hold on the center of the target counts as a center hold. My reservation, like I said, is that having the entire bull, from the 7 ring inwards sit on your front sight seems a little imprecise–this could also be a result of the extremely short distance I shoot at which makes the official bull larger.

    Your comment about the adjustable sights makes sense as the solution. I’ll take another look at my Daisy 747, but I don’t recall any means of adjusting the width of the sights, front or rear. You can adjust the rear sight for windage and elevation and that’s it–one price you pay for buying on the low cusp of the legitimate. I guess I’ll plug on for the time being with my customized dots for targets and an improved understanding and wait to see what options appear in the series on match pistols.

    Yes, those videos are fascinating. Those shooters were like Terminator–so frozen and statuesque that I couldn’t even tell when they touched the shot off.

    Thanks.

    Matt61


  24. Okay, all of you technical wizards, how about this for a JFK style moon landing challenge.

    I think it would be a great teaching tool if someone could somehow produce a shooter’s eye view of a shot through sights–open or scoped although scoped would be easier. I haven’t seen anything like this. The G. David Tubb video on his one-mile shot was full of information about his rifle and the ballistics, but when it came down to the shot, all you saw was him in the prone position. He was wiggling around in a way that looked very detail-oriented, but it didn’t look very different from what I would be doing although he was hitting the target and I probably would have missed by about a mile. With a scope view, you could see how much movement was there and how it was dealt with. You could even combine this with a voiceover where the shooter could explain what he was thinking about at the time such as how he was timing his hearbeat. Heck, you could do a split-screen showing the trigger to display how the trigger squeeze coincided with the sight picture. This would seem to be worth a lot of footage from the outside of elite shooters reeling off these incredible shots like nothing.

    It doesn’t seem like it would be that hard to do although I’m not strong on the details; that’s where you come in. It looks like the central problem is getting a good picture of the scope without disturbing the shooter’s view and shooting position. Maybe you could rig up a fiber optic cable that could wind around the shooter’s head to capture the image. Or doesn’t the military have one of those devices that soldiers wear on helmets that transmit video of what they see back to the Pentagon? Or maybe you create some kind of periscope device that redirects and enlarges a reflection of the sight picture to where it could be put on film. Or maybe as a low-tech solution, you could somehow mount one of those miniaturized digital cameras on the stock and take your shot through it.

    Or maybe someone has done something like this already and you can point me to it.

    Matt61


  25. Derrick, I used green vinyl gardners tape and a punch set. The tape has the ability to compress somewhat. Thats my quick fix if I dont have aluminum or plastic laying around.
    –Dave Ennis


  26. Dave,

    Thanks for the tip. Judging by all my wife’s garden center receipts, there’s probably some of that tape in the garage.

    Matt61,

    Yep, I think you’re right. The 747 has adjustable sights, just not that adjustable. Don’t despair. The Gamo Compact doesn’t have an adjustable width front sight–and it’s really on the thin side for good 10 meter work. Unfortunately, the Gamo front sight is molded right into the barrel cover, so changing it would be complicated. The rear sight on the Compact is width adjustable for the notch opening, but it’s actually not quite wide enough in overall width. If the dollars permit, I’d skip over this pistol and get a 46M or better.

    The Baikal comes with several (maybe three?) different front sights all marked in fractions of mm’s I think I use the 3.4mm for 10-meter targets. My gun also came with 2 or 3 different rear sight “blades” that had different width notch openings.

    The quality of the sighting alone is worth the price difference.

    You might also want to look into a used Feinwerkbau 65 pistol. I see them all the time in the high $400 to mid $500 range. If nothing else, get hold of one and point it at something to see what the sights “look” like. They exude precision.

    Derrick


  27. Vince, I tried shimming the breech of my Norica, and I was very pleased with the results. After installing the shim, the original O-ring protruded a bit and ripped a tiny chunck of it on the bottom side (inside toward the port). It was just enough to shave it off with a razor. Other than that, I’ve had no problems. I will let it sit for a day to see if the O-ring tightens (squashes) into the shape of it’s new position against the air chamber port. I thought that this would effect the accuracy (shooting lower, possibly increasing any barrel droop), but it didn’t seem to at all. I don’t have a chrony, but I will say that the pellets hit/sounded quicker than before. Even more evident at my 35 yard trap!!! Thanks for the blog, Thomas


  28. Hello all. For measuring the o-ring groove ID and OD plus the thickness of the current o-ring please buy an inexpensive digital caliper from Harbor Freight or similar. Usually under $20 and you will use them far more often for things than you can imagine.

    Match the dims and use the McMaster catalog to match the o-ring size. If at all possible, try to use rings made of Viton as they do not tear as easily and generally have a higher durometer than the regular rings. The minor higher cost is well worth it. (I’m a believer of doing the job once and right. Pure lazyness, or time management as my wonderful bride calls it! Oh dear, am I opening up a bunch of questions for o-rings now?


  29. BB,

    Sorry off topic. I would like to ask if you have blogged about the Logun S16s pcp? I think I have seen your s16 CO2 conversion but I could not find one on the full power pcp.

    Thanks.

    David


  30. Vince,

    Thanks! Great blog! Good pictures and narrative to get your meaning across. I haven’t shimmed any of my guns, but I think I have a couple of candidates for it.

    Dave Ennis,

    Thanks also for the tip on the P17! I hadn’t thought of shimming that one either. Maybe it’ll bring me up into the 400′s too, despite the mile high elevation.

    /Dave


  31. BB,

    Thanks for the info on the HW57 seal. It seems to be holding 100 more fps out of the original 140 that it gained. Maybe it was just a bad fitting piston seal that the detonations fixed. I try not to dry fire any of my guns, but this one got me by surprise with that odd loading port. They should have connected it to the underlever to close automatically when the lever is returned to its starting position. I’ll have to look into this to see if it can be done. Until then, I’ll just be more careful…

    /Dave


  32. Derrick,

    The forces are mobilizing for another Russian gun purchase. I hear good things on all hands about the IZH 46M. My skills will need significant improvement first, though.

    Matt61


  33. Hi Vince,
    Would like to compare notes with you. Here’s what happened after I shimmed my .22 34 Panther.
    Before After
    cp wads 650 fps 756 fps
    hp 663 764
    Daisy wads 645 733
    Rws super d 652 766Hobbys 704 801
    Above, they are averages of 5 shots. I’m now satisfied with the after figures as it now has the speed for pest control up to 30 yards. Thanks to you.

    Hank



  34. Matt61

    Matt, ah, you’re not fooling me here. I’ve got a feeling that you’re a pretty darn good shot already.

    If you like your 747, you’ll love a 46M.

    Derrick



  35. BB,

    Yes sir, the Logun S16S was not yet tested and reported here with compressed air. Looking forward to reading your blog on it.

    David


  36. VINCE and HANK

    Would you mind sharing the thickness of your optimal shims for the RWS 34 panther? If thickness unknown, then what the shim is made from (brand of coffee lid, etc)

    Thanks, Chris


  37. David,

    I sent the S-16S back more than a year ago. I won’t be doing a report on air. All you need to do to see it on air is push those groups out about another 20-25 yards.

    Please remember what I said about that HORRIBLE trigger! For some reason, I seem to be the only writer who is bothered by that, though everyone privately admits the Logun trigger is way too heavy.

    I could never cotton to a rifle with a 10-pound trigger. So testing the S-16S again would be a waste for me. That’s probably why I didn’t test it on air the last time. I just gave up on it.

    B.B.


  38. Vince,
    GREAT blog… very practical and elegant fix to what seems to be a common yet until now undiagnosed problem.

    B.B.,
    Was just thinking and wondering if shimming might fix the Beeman SS100H 22 barrel problem. Sounds like such a problem could possibly be the gremlin in that rifle.

    DB


  39. Chris,
    Hope Vince & Hank don’t mind but I’ll offer you an answer to your question about shim thickness.

    Answer is each gun may need no shim or a very thick ship. What the shim is doing is compensating for a machining flaw during production. Seems the groove for the O-ring is sometimes cut too deep. How much to deep will very… this the need to experament and see what works for your gun.

    That is my take on it anyway.
    DB





  40. This is genius! Took my RWS 350 from mid 820′s to mid 860′s with only a couple minutes work (and I don’t even know if you could call it work with how easy it is). Thanks! :)


  41. Nick,

    Assume you shimmed your breach seal to increase your velocity?

    Aren’t these “airgunning secrets revealed” exciting?

    Your comments and first hand experiences should be shared with airgunners like you that are just as interested in these revelations. The comment you posted is attached to an article that is almost a year old. Please join other airgunners as enthusiastic as you are on the current/active comments section of this great blog. Here’s a link to take you there:

    http://www.pyramydair.com/blog/

    kevin


  42. Beeman sez some blowby is "normal" (implying it's a good thing).
    You shouldn't be able to feel anything blowing out of the breach if seal pressure is greater than air pressure.

    As for chronographs, here's a free one. Record the firing & strike sounds to a target of known distance in a sound editor. Looking at the graph, highlight the area between sound spikes. The number of samples in this area is the total time in sample periods. Example: 22KHz sample rate = 1/22,000 = 45.4 microseconds/sample; 62 samples would be 2.82 milliseconds. Of course, you have to subtract the time (in samples) for sound to return from the target, but the rest is flight time.
    I said it was free — not simple ;-)


  43. Mark,

    while we have quite a few regulars on this blog that have no problems with math even more advanced than this (including one honest to God rocket scientist), a chrony is more convenient to use when trying to record different pellets' speeds to see what works best in your rifle. It's especially useful for determining the sweet spot of a charge in a PCP where you have to track 30 or more shots. However, your method is quite ingenious.

    Regarding shimming the breech seal, I have a RWS 350 that was shooting a 10.5 gr pellet at 745 fps and producing under 13 ft. lbs of energy. I only found this out with a new chronograph I had bought. Looking up what this rifle was capable of on the web, I found I should be getting closer to 18 ft. lbs. Long story short, I found the seal was not sealing well and after shimming it, the same pellet traveled 835 fps producing 16.33 ft. lbs. An RWS R-10 8.2 gr pellet went from 860 to 995 fps and 13.5 to 18 ft.lbs. respectively. I don't want any blowby, preferring all the air pressure going behind my pellet.

    By the way, you have posted your comment to a blog written 2 years ago. Few people other than a small core of volunteers monitor these blogs, I being one of them. The blog is written on a daily basis, Monday to Friday and can be found here:

    http://www.pyramydair.com/blog

    You will get many more comments and information if you post on the current days' blog. Off-topic comments or questions are always welcome.

    Look forward to seeing you there.

    Fred PRoNJ


  44. Mark, ditto on comin' over to the current blog.

    I actually tried the sound-recorder chrono (over a distance of 10')… and the results were ballpark, but not great. I had the mic equidistant from the muzzle and the target. One issue might be that the worst of the firing noise doesn't necessarily coincide with pellet exit. Also… you can't really get muzzle velocity this way, since you need a significant flight distance.


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