by B.B. Pelletier
In this third report on the air transfer port of a spring-piston air rifle, we’ll look at the optimum port diameter. I actually have data from the Beeman R1 that Dennis Quackenbush converted to take different air transfer ports, and I have some data from my Whiscombe JW75 rifle that came with transfer port limiter screws. We’ll look at the R1 data first.
Testing the Beeman R1
The ports I have for the testbed R1 range in size from a small (0.075″) to large (0.160″). In my testing, I discovered that a port size of 0.110″ limited the velocity available in the .22 caliber barrel shooting .22caliber Crosman Premiers by about 50 f.p.s., and a port size of 0.125″ gave the highest velocity recorded. A larger port of 0.145″ gave identical velocities as the 0.125″ port, but 0.155″ was about 20 f.p.s. slower. The optimum range of port sizes for a .22 caliber Beeman R1 falls between 0.125″ and 0.145″.
Switching to a .177 barrel on the same R1 and shooting .177 Premier lites, the results were fairly close to the .22 barrel, except that the 0.110″ port lost only 40 f.p.s. and the 0.155″ port might have lost slightly more than 30 f.p.s. Because I was averaging a spread of 10 irregular shots, the numbers are not as close as I would like for a reason I’ll mention in a moment.
The conclusion I drew from the test was that for the Beeman R1, a port size of 0.125″ is ideal for both .177 and .22 calibers. That happens to be the size Weihrauch uses for that model. I would assume .20 caliber would also work well. Since only a few .25-caliber R1s were made, I’m guessing the factory left the port size at the same 0.125.” That makes their job easier and also allows for the field swap of a different-caliber barrel without a major disruption.
But wait
Now for the downside. The transfer ports Dennis made leaked a little. They were a close fit in their hole in the compression tube, but they had no special provisions for sealing their outside diameters other than the closeness of the fit. It seemed that some air blew past the port on every shot. The velocity was about 50-60 f.p.s. slower in .177 caliber and 30-40 f.p.s. slower in .22 than an unmodified R1, but that’s difficult to pin down since this testbed rifle can be fired only with a replaceable transfer port. Since no two rifles perform the same, it’s a stretch to compare the testbed rifle to an unaltered R1. I don’t know if this affected the outcome of the test.
A second test rifle
The Whiscombe that I made into a 5-part report also came with transfer port limiters that operate exactly the same as the replaceable ports Quackenbush made for the R1. They fit tighter, however, because the restrictors are Allen screws with air holes bored through their centers. The screw threads, being very close in size to the corresponding threads in the end of the compression chamber, offer greater resistance to escaping air than the straight sides of the Quackenbush ports.
Though I didn’t do a test of comparative velocities per se, I did write in Part 2 of the report that the rifle it was shooting .22 caliber Crosman Premiers at well under 12 foot-pounds when I received it. When I removed the Allen screw transfer port limiter, the same .22 caliber rifle shot Premiers at 26.88 foot-pounds. You can also read in that report that the transfer ports of various sizes affected velocity much more than the Quackenbush ports on the testbed R1.
Evaluation of a third spring rifle
I also measured the transfer port of a Webley Patriot and found it to measure approximately 0.154″. I say approximately because I didn’t disassemble the rifle to take the measurement. Clearly, this port is much larger than an R1 port, but it had a strange affect on velocity. In .25 caliber, it worked well but in .177 caliber (Quackenbush sent me a Patriot with all four caliber barrels) it couldn’t even break 1,100 f.p.s.! I found that shocking until I looked at the R1 test data. The .177 caliber did not like the larger ports there, either, so there must be a caliber relationship to port size.
Conclusions
This really isn’t enough data to form solid conclusions, but it appears that as caliber increases, the transfer port size can also increase a little without power loss. And a smaller caliber can more easily tolerate a smaller port.
The second observation is that the port works best when it’s the only place the air can pass through. Any blowby and the advantages of the perfect port size might be lost in the overall inefficiency of the rifle. That would include a weak breech seal.
Gamo’s and the BAM built copies (B19 and Quest variants) all have a .125″ port. I played with a number of these and always found that opening the port to 5/32″ (.156″) virtually always gave me another 20-40fps. But only on .22 – .177 didn’t seem to care one way or another. Opening the port further to 3/16 didn’t gain anything in .22.
I once tried the same thing on a .22 B26 and saw no improvement at all, which surprised me since the powerplant specs seem generally similar to the B19. So I suspect that this sort of thing can only be determined on any particular gun by trial and error. And I also suspect that pellet weight plays into it as well.
Vince,
Thanks for sharing your data. Yours is the first port-size data I’ve seen, other than my own.
I believe you’re right that each gun responds slightly different to port sizes, and I think I know why. Each of them may have a slightly thicker or thinner compression chamber plug through which the port must pass, and that would determine the port volume.
B.B.
B.B.,
What differences in sound are produced using the same caliber with different port sizes?
I wonder if the Whiscombe is quieter when using same caliber and shooting at 11 vs. 25 ft. lbs., and by how much?
Using the same power, does .177 sound noticeably different than .25?
– Dr. G.
BB – I believe the Cardew’s research showed the same data – .125 seemed the ideal port size. They speculated that the additional air volume provided by the larger ports was the cause of the reduced velocity of the pellet (more room for the compressed air to expand as it got to the pellet). FWIW, this might be one reason for the ideal port diameter. It would be nice to hear the folks who specialize in fluid dynamics (OK, rocket scientists) studies to add their thoughts.
Dr. G.,
Involved question – simple answer. The smaller caliber always sounds quieter – at least to my ears. Port size has nothing to do with it. It’s the bore size that limits the sound.
But understand that we are talking about very small differences because spring guns are inherently quiet to begin with.
This carries over to PCPs as well.
B.B.
BB,
It makes sense that the larger calibers can benefit from larger port sizes on big enough power plants. Did the size have any effect on firing behavior? I was assuming the smaller ports would have been smoother up to a point, i.e. until they became so small that the piston bounced excessively or there was still air coming out when the pellet left the barrel. The Whiscombe with power limiting plug would probably answer this question, although it may be too smooth by design. Even on a magnum, losing, e.g., 40fps might be fine, if it made it less hold-sensitive or easier on scopes.
Were the Quackenbush ports sealed with loctite, teflon tape or pipe dope?
B.B.,
Caliber size would of course have two primary effects.
First the nominal weight of the pellet changes. I would think that for the lightest 0.22 pellets that the best transfer port size would be larger by a step. For a heavier pellet I’d think that you’d go down a size.
The reasoning is that a lighter pellet accelerates faster and will leave the barrel more quickly. Conversely a heavier pellet is in the barrel longer, so a step down would likely be optimal.
The other factor in caliper size is the volume of the barrel. Imagine a PCP with a pressure chamber and a plugged barrel. When the fixed volume of gas gas at a given pressure is released into the the barrel, since the overall volume increases by the volume of the barrel, the pressure drops. Since the 0.22 barrel has a greater volume than a 0.177 barrel, the pressure drop is grater. Thus for the same weight pellet, the 0.177 barrel will be faster because the barrel volume is smaller.
Herb
B.B.
It seems like this is a very important issue, and the tech guys should have studied it more before building a new airgun.
I guess the marketing dept. is just too strong to let the tech guys work on it long enough..
I like the allen screw as the port, idea best. It seems like a great way to try different sizes and shapes, while limiting the blow by effect.
Anyway, some company should really test this further, and get a real advantage in the market place.
Wayne,
Ashland Air Rifle Range
BG_Farmer,
I didn’t record how the firing cycle feel changed.
The ports were not sealed in any way.
B.B.
Wayne,
I’m not sure that airgun companies have not already tested port sizes. 0.125″ is a very common size, so they either tested it and liked what they saw or they copied other guns they felt were successful.
B.B.
RE: Sound and Pellet size
Seems like sound would be higher pitch for a smaller diameter barrel. Think church organs. Larger diameter pipes have a deeper sound.
Could this also explain why pumps/PCP power plants are nosier? There is some pressure left in pressure chamber as pellet exits barrel. With sudden release of pressure the air in pressure chamber blows through transfer port creating an air vibration down the barrel.
I wonder what kind of noise would result if you just blew 200 psi air through transfer port into an empty barrel.
Herb
Good morning B.B.: I’m not a real tech guy although I really appreciate the fruits of their labors. The Whiscomb rifle with its wonderfully timed power plant. A Ducatti with its desomonic valve train. I have been waiting for this report and will be reading and thinking about all your comments. I wonder what Miss Jane has to say and hope she will be a contributor to this topic
B.B.
Could the different port shapes mentioned in the previous blog on this topic have affected things?
All, believe it or not my B30 has tanked again. While shooting away, the cocking lever developed a cracking sound as I worked it, and some large mass shakes around in the region of the compression chamber when I shake the rifle back and forth along the barrel axis. Any ideas what this is? Was there ever such a rifle? It shoots great and has kept shooting great even with this latest malfunction, but things keep falling off! Well, I’ve had enough. No more Mr. Nice Guy! I’m summoning Rich from Mich to deal with this based on recommendations on this blog. Reading his site made me feel better. He describes a very thorough process of custom deburring, fitting, lubricating, replacement with a custom spring and a trigger job, all at a very reasonable price with a turnaround within a week or so. Talking to him is even more reassuring; he inspires confidence. He claims that he has a couple of these rifles and that they can be made to shoot well. So, it looks like I can retrieve the day after all with a good-shooting rifle that is still less than the RWS 48; that’s not including all of the hassle. I’ll report on how it turns out.
Herb and BG_Farmer. How about this for the MOA question. My point was that I don’t see how a rifle could reduce MOA over distance. At most, a rifle could maintain MOA because once a pellet starts moving sideways, inertia will keep it moving even after the original force that started the motion is removed. But that assumes an ideal case without air resistance. I originally decided to ignore air resistance because the sideways dispersion rate of the pellet is so tiny that it wouldn’t create air resistance on anything like the scale of the air resistance caused by the forward motion of the pellet. However, at high speeds, especially supersonic ones characteristic of firearms, air drag becomes very different, so that we can say, at the very least, that the ideal case no longer applies. Once the inertia of the sideways motion can be retarded, the constant MOA limit gets erased and you can have a reduction in MOA.
I still don’t see that stabilization of the projectile in the form of a reduction of a widening concentric spiral around the point of aim is the sole explanation. But undoubtedly, the reduction of a spiral path along with whatever happens with air drag at high speed is enough to explain things. I guess you could say that I’m chipped away. And I’m always ready to privilege experiment over theory anyway.
Matt61
Matt,
That sounds very much like a broken spring to me. Your report on the conclusion of this episode could be a guest blog.
B.B.
Matt,
Sorry, I don’t really understand your point. But you can’t ignore air resistance. The air makes all sorts of odd things happen.
The air resistance doesn’t just retard forward motion. If that were the case then you’d get a vertical oriented grouping.
Again, in order for the MOA to decrease at distance, there must be some factor initially causing dispersion which diminishes as distance traveled gets longer. For instance there is a puff of air as the pellet leaves the barrel which pushes against the back of the skirt of the pellet. This in turn will cause the pellet to precess. But the forward motion causes air resistance, which in turn not only causes the pellet to slow but also dampens the precession. So the imprecision due to the puff off air diminishes as the pellet gets further away.
Took me a while to get it, but unlike a bullet, the pellet depends heavily on the skirt to stabilize the flight. A bullet depends on spin alone. A pellet spins some, but very slowly compared to a bullet. Not spins per foot, but spins per second.
Question: Is there a BB/pellet gun which has an absolutely smooth and straight tube as the bore (Daisy Model 840 Grizzly G ?? )? My guess is that BB’s would come out like knuckle balls and have a much larger dispersion pattern than pellets. RWS Superdome and round balls (0.177) weight almost the same. Be an interesting test.
Herb
Herb,
I had an older smooth bore Diana that I thought would be better of shooting the Gamo round lead balls. Wrong again. Almost all pellets out shot the Gamo balls in the smoothbore.
Also, at ten meters its only handicap was no provision for a scope.
Volvo
Matt,
You’re getting there, but we can’t ignore anything really. Even when we collect everything we know, it probably won’t account for everything:).
Consider that a spinning cylinder actually creates a force in air, even apart from precession and nutation…if I remember correctly there was a wacky 70’s “Popular Science” feature on sailing ships with powered sails that work in that fashion (Magnus effect). Try this link for memorable pictures (I’m not otherwise endorsing it):
http://www.pilotfriend.com/photo_albums/potty/2.htm
Now, Herb brings up a big question: pellet or bullet, i.e. “drag-” or “spin-stabilized”. For bullets, the spin definitely comes into play.
Herb & Volvo,
I've wondered whether a good smoothbore would suffer any against a rifled bore with low speed pellets. The main problem in finding out is that most high-quality pellet guns are pellet rifles, with the smoothbores at the bottom tiers, so there are too many other uncertainties. I'm pretty certain FWB is not going to waste resources creating an unrifled 10M machine even if it should work: acceptance would be a hard sell.
B.B.
Can a rifle still shoot with a broken spring? This one did. I’ll be eager to hear the report from Rich about what he finds inside the rifle. A guest blog might bore people since they’ve heard this story in the comments. But I’m up for it if you think it’s worthwhile and can tolerate blurry pictures.
Herb and BG_Farmer, as I understand it physicists cannot even say for sure how a bicycle works so an exact description of what is going on with a flying pellet/bullet not to mention the internal workings of airguns seems to me overwhelming. I was just puzzled at how the sideways inertia of a pellet could be stopped once started and having satisfied myself that this very basic law of physics is not being violated is enough for me.
Matt61
BG_Farmer,
I have two things to add to this discussion. First, the Daisy 499 is a smoothbore and is as accurate as any rifled match gun at close range.
Second, there was a group of shooters in Ohio in the middle 1800s who shot round balls from smoothbore guns. They apparently got remarkable groups at long distances (100 yards was a long distance for sporting guns in those days). I don’t have much on them and I have misplaced the reference to them, but I think they were getting two-inch groups at that range for five or more shots.
Okay, I have three things. In the 1960s when the French Balle Blondeau first hit these shores, shooters were astounded by smaller groups from a shotgun barrel. Today a two-inch group at 100 yards seems possible from a shotgun and a modern rifled slug, but back in the 1960s it was unheard of.
Gary Barnes used the Balle Blondeau pattern for many of his big bore bullets that he called dog bones and flying dumbbells. They will group remarkably well from a smoothbore at distance.
B.B.
BB,
Thanks! I didn’t mean to exclude you from the discussion, just didn’t want to add to your workload:). It sounds like drag stabilization does work. Do you know if rifling was introduced to pellet rifles as an informed designed decision or was it just a given, i.e., the most accurate firearms use rifling, so the best pellet guns should as well? My main interest is that the spin rate is so slow it seems like it may actually cause more problems than it solves, but I’m just speculating…
Somewhat related, I’ve been pretty impressed also with the groups people are talking about from smoothbore slug guns and ordinary slugs. Not sub-MOA, of course, but the shotgun as a “deer rifle” was almost a joke when I was a kid. Makes me question our ideas of musket battles.
I have a Daisy 499 that does extremely well out to 5 meters or so. That was my reasoning when I acquired the Diana smoothbore, feeling it would need lead balls and not pellets.
Perhaps the difference is the lead balls needed jammed in the barrel of the Diana distorting them so they were no longer perfectly round?
The precision hard shot the Daisy uses just rolls down from the muzzle in contrast.
I do know even cheap Daisy max speed pellets out shot the Gamo balls.
Bg farmer, I think many of the post WWI pellet rifles had to be smoothbores. Once bitten twice shy.
Matt,
Often people shoot for years with broken springs. It’s not a big deal until is makes the noise you heard.
B.B.
BG_Farmer,
I think those accounts of musket inaccuracy are valid because the balls were so much smaller than the bores. They did that to be able to keep shooting as long as possible without cleaning.
You remember the same things I do about shotgun slugs. These new slugs have changed the face of a hunting shotgun.
As far as I can tell, rifling was introduced to pellet guns by BSA. I think they did it as a given, because the firearms they made were rifled and they knew the advantages.
The thing I have never seen is that anyone back then (1900-1910) put it all together and realized that the diabolo pellet was just as important to accuracy as rifling.
In fact, it kinda makes me wonder whether a large diabolo might not work very well with a muzzleloader.
B.B.
RE: Knuckle ball
Really the wrong analogy. A knuckle ball picks up its spin in flight. But the direction that it will spin is unpredictable.
A ball would pick up its spin from friction bouncing around in the barrel. So it would be spinning as it leaves the barrel, but the spin would be in a random direction in a smooth bore.
Question – Somebody must have a cheap gun in different versions – one which has smooth barrel and other rifled. (Crosman 760 & 760 XLS might be another possibility…)
Herb
B.B.
Surely, you don’t mean a broken mainspring that propels the pellet. How could you possibly get enough force to shoot with a broken mainspring? How many other springs are there in a rifle?
As for the new shotgun slugs, their improved performance must be based on shape, right? What was the big breakthrough?
Matt61
Matt,
That is exactly what I mean. Springs seldom break in the center. They usually break off an inch on either end. They get weaker, but not as much as you might think.
The big slug breakthough is the high-drag tail. It stabilizes the slug the same as fletching on an arrow.
B.B.
BB,
Thanks for the informative research.
Did you happen to record the length of the transfer port, or the total length from the end of the chamber to the beginning of the barrel?
I kinda suspect that the longer the distance to the barrel, the smaller you want the transfer port to be.
B.B.
Wow, amazing. Those springs must be super powerful. What happens when you put a bullet-shaped projectile into an air rifle? I seem to remember this being done with the Condor. Is this not done otherwise because airguns don’t generate enough force for this shape?
Matt61
Hi BB, I was hoping you could answer this question: what are the lowest weaver rings that pyramyd sells? I need the absolute lowest ones for my 397 with air venturi intermount. i think the B-Square 1in ones are lowest, right?
Has anyone seen this rifle or know anything about it? I think it's one of the nicest guns I've seen. The checkering is really cool.
http://www.norica.es/index.php?opt=2&fam=26#
BB- any chance of walther talon this week?
John
BB,
You’re probably right about the muskets — and it does seem pretty well documented…I also seem to remember something about the balls having a flat spot (whether intentional or from the ramrod, I don’t know), which may not have helped either.
My current candidate for best guess is that pellet shooting guns are rifled because they developed from rifled firearms in similar applications (e.g., the Zimmerstutzens) and the theoretical aspects were never examined rigorously. Sounds like what you are saying probably happened with BSA.
There is a small doubt in my mind, though, that the spin of a pellet, while not enough to stabilize it (even if it were the right profile and size) may buffer against anomalous effects such as the muzzle turbulence Herb mentioned, i.e., that even spin-induced understability (I’m assuming) is preferable to the more unpredictable alternatives.
Matt,
My clunk (36-2) shot progressively better as spring bits broke off, until it was about as powerful as a Red Ryder. Maybe consider asking Rich to bias tune towards shootability rather than power output. As heavy as a B30 is, it should shoot very nicely if detuned a little.
I also doubt we can understand everything, but even now we can make better bicycles.
Volvo,
I wonder if they make BB-sized diabolo’s? 499 would be a good test bed, within its limitations.
Matt61
When the spring broke in my Shadow 1000 I couldn’t feel much difference. I didn’t know what it was at first but a few weeks later the gun would not cock. You could break the barrel all the way but the sear would not engage.
I got a tune kit from Rich, it included a Maccari spring, both spring guides, and all the lubes. I also bought a piston seal,rollers, and slides for the cocking arm. It shoots and cocks much smoother now. Mine didn’t gain or lose power.
jeff
BG_Farmer,
You’re right about bicycles. The history is amusing to read and clearly driven by trial and error. Do you suppose that the stabilization of a projectile is a threshold effect where you need a specific rotation rate to get any effect? I had always imagined it as something gradual where every little bit helped on the way to the optimum. Power has never been a consideration of mine with the B30. I just want the gun to function reliably. I’m so curious about
Rich’s diagnosis of the source of the continuous problems with this gun.
Jeff, you’re a brave man to get the tuning kit, and the price is certainly right. But I think my gun needs the attention of the master himself. Besides, it’s fun to meet these master gunsmiths.
Matt61
Transfer port length,
No, I didn’t record the length of the port, but I’m pretty sure it matters greatly.
B.B.
Matt,
A bullet must be spin-stabilized, so the velocity of the rifle that fires it and the twist rate of the barrel determines the rate of spin – always given in revolutions per second (RPS). To stabilize properly a conical bullet ( the correct name for a bullet that is not spherical and it longer than it is wide) must spin fast enough to not tumble in flight.
The twist rate for a .22 Long Rifle is one turn in 16 inches of barrel – written as 1:16. .22 LR ammo varies in velocity from 1,000 f.p.s. to 1,450 f.p.s., and the bullets weight varies from about 37 grains to 40 grains.
A .22 short barrel twist rate is 1:22 and the 29 grain bullet leaves the muzzle at between 1,000 f.p.s. and 1,100 f.p.s. Therefore, a .22 pellet rifle must perform the same to stabilize the solid pellet (i.e. bullet) it shoots.
B.B.
ShootThePigeons,
The B-Square rings look lowest to me.
B.B.
John,
Thanks for the reminder. I’ll start it tomorrow.
B.B.
RE: Spin rate and stabilization
https://oa.doria.fi/bitstream/handle/10024/2120/woundbal.pdf?sequence=1
Page 22/23
Probably not the best discussion on the web, but it gives you a starting point for the factors involved.
Even though the pellet rifle has the same twist as a 22 rifle, the projectile is slower so the spin rate in rev/sec is slower.
As a scientist it is typical that no one factor is the sole source of error. Add a little for this and a little from that to get the overall error. So no spin is bad, but the perfect amount of spin won’t over come all the other error sources.
If you have one source of error that contributes say 80% of the overall error, then “fixing” that factor so that it is 1/4 of its original contribution will result in a significant overall improvement. But if you have four factors, each which contributes 25%, then it is hard to fix any one thing and see an improvement. So typically one or two iterations through one factor at a time and you hit a wall.
Not only do factors interact, but measuring one in the presence of the errors from the others is hard.
Herb
fwiw…
the transfer port on my 177 BSA Lightning XL is ~0.125. The port on my 0.25 Beeman Kodiak is ~0.150.
BB,
I made up my mind. Obama won and I plan on buying a lee enfield Mk4 (or wich ever uses the largest loads) and a 20ga super black eagle II semi auto. Any advise on matenance for the lee enfield, what generally breaks after time, what loads are appropriate. I might even hand load again, dads moving to Australia(stricter gun laws, but he gave me his guns).
shadow express dude
shadow express dude ,
The Mark IV is a softy. I shoot factory ammo in mine and there is little recoil. Remember that lighter bullets mean less recoil, so get the 150-grain bullets when you can.
What breaks is the extractor spring. Buy three extras. Everything else is pretty tough.
B.B.
Herb:
Re: Knuckle Ball Analogy
If you read the heading of the list, it was “Ballistics 101”, not Advanced Aerodynamics. And if you re-read your post, the knucle ball and the musket ball differ only in where the spin starts.
Hi BB, I was wondering if you could tell whats wrong with my 397 owners manual. Pyramyd air says to store it with 1pump of air, but the owners manual doesnt mention it. I dont want to kill my gun, so what should i do?
Thank you
Pigeons,
You’ll kill your gun if you DON’T store it with a pump of air.
B.B.
Slightly off topic, but I recently acquired a Daisy 488B BB gun. It’s in working order, and good shape considering it was being thrown out. I was wondering what you could tell me about it. Its lot # 3A1990. I’m wondering a few things about it, like what year it was made, what kind of sights I can put on it, and what I can expect from it in regards to accuracy and power. I don’t have a crony, and I’m trying to get a feel for if its up to par. Thanks for your help with this, if you would like pictures, let me know.
Randy
Randy,
I’ve never heard of a Daisy 488. Did you mean a 499?
If so, read this:
/blog/2008/05/daisy-avanti-499-champion-part-2-the-worlds-most-accurate-bb-gun/
B.B.
Yeah, I meant the 499, sorry about that. Any idea what the B at the end of it means? Thanks =)
Ballpark,
The 499B is an updated version of the 499. It has some plastic, where the 499 was all metal, I believe.
Contact Daisy for date info.
The velocity with Precision Ground Shot is supposed to be 240 f.p.s.
B.B.
It's similar to SCUBA cylinders I suppose, if the pressure in the tank is too low it allows respiration, drawing air in and out with pressure and temperature changes, and consequently condensation which will lead to corrosion inside the tank. Storing it with pressure higher than the ambient pressure prevents this respiration and pushes the valves closed.
Lee Enfield No4 Mk1* is the best of the bunch, uses the same loads for all models, 173 grain boat tail full metal jacket is the round designed for it, 180gr soft point are the closest and best legal hunting round to suit it.
The differences are pretty minor, all interchangeable parts between models, most major difference is in rifling, no4 mk1* has a five groove rifling as opposed to 3, and 2 in some of the other models, designed for snipers, it is the cadillac of the series.
The only thing that ever broke on mine was the firing pin, after many years of trudging through rain forest.
I've seen 80 of these in service for 11 years, never saw another broken one. I think one the rifling was worn out, bullets tumbling and impact on target sideways… after 65 years of service and a few wars, not too surprising!
A friend had a sport model, left it in the bush for a few years, pulled it from underground, weeds growing right through it, and it fired, (dummy) I cleaned it up for him looks perfect, works perfectly, very tough, reliable and accurate up to 1000yds easy. I would buy one without hesitation, no matter what condition even if just for parts to build one.
I realize this is a very old thread, but fluid dynamics are a major part of my day job so I thought I’d add a bit to the discussion as it relates to blow-by in the “interchangeable air transfer ports” (venturi) and the shape of the venturi at each end.
– Floating venturi (meaning no threading or subassembly for locating and retaining its position) that are experiencing blow-by are not, and cannot be, centered in their seat. In round venturi that are positioned horizontally (as in an air rifle) the center is almost certainly going to be below the central axis between the piston and the pellet.
Meaning the air is first going to push the pellet nose up, BUT, because the venturi is not properly seated and sealed on its OD it will move as air travels through it. Like a water hose if you hold it a few feet behind the exit end. This effect is exacerbated by the blow-by.
This action transfers to the pellet itself, where it’s largely planed out by the barrel, at the expense of energy that could have been used pushing the pellet straight down the barrel. Given the low energy budget of a firing event the losses can be significant.
Below only matters if blow-by is not happening.
– The shape at each end of the passage through the venturi can have a significant affect on the way the air behaves. Beyond simple geometries like the funnel at the beginning of the article, the way those geometries are finished is important in managing flow. If the discharge side (closest to pellet) of the venturi is simply milled square and the edges of the actual orifice are at right angles you introduce turbulence that has an affect similar to what was described above but of a lesser magnitude.
Chamfering the edges of the discharge orifice helps reduce that and it’s something you can do with a drill press and a countersink. Rounding off the edges instead of chamfering will work better, but that’s more difficult as doing it well requires tools most people don’t have access too.
What’s the real world difference between square edges and those that have been smoothed and refined? Small, but definitely measurable, and it’s the sum of a bunch of incremental changes that make a gun better, not just one or two things.
Welcome to the blog.
Thank you for your explanations. They will no doubt spur more interest in this subject.
B.B.