GTA
All Springer/NP/PCP Air Gun Discussion General => Air Gun Gate => Topic started by: rsterne on April 09, 2012, 01:52:52 PM
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On another forum I got into a discussion about barrel length and accuracy.... A chap who used to work for BSA stated that if the barrel is too short the pellet spin rate would be too low for stability.... I ran the numbers, and of course if you shorten a barrel to the extreme the velocity is so low that the pellet RPM is too low for stability.... that makes sense.... He stated that was NOT the mechanism, however, and that the "velocity peak and the pellet spin peak" occurred at different barrel lengths.... He stated that the velocity peak occurred as much as 8" before the pellet spin rate peak....
I can't see how that is possible.... Surely the pellet, once engaged into the rifling, accelerates both rotationally and horizontally at the same rates.... double the velocity, double the RPM.... To do otherwise would smear the rifling marks on the pellet.... Am I missing something?.... Is there some mechanism that would cause (allow) the pellet to "spin up" at a slower rate than the rifling says that it should?.... We're not talking progressive twist or smooth twist barrels here, BTW.... a standard, constant, helical rifling twist....
Bob
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I am no genius or even in the same class as you.
When Looking around at different air rifles... the twist rate has been usually between 1-12" to 1-15".
Maybe this is what he means.
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If what he states is the case it would seem severe leading would occur as well? I mean if the pellet is plowing through the bore without spinning with the rifling i would think it would strip alot of lead off the pellet and deposit in the bore
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Considering how fast the pellet accelerates, wouldn't it be difficult for the pellet to reach its full RPM right away?
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I am no genius or even in the same class as you.
When Looking around at different air rifles... the twist rate has been usually between 1-12" to 1-15".
Maybe this is what he means.
From what I know about ballistics, is just this, (from firearms.)
From what I have learned about pellets, there is a max rate of fps for pellets until they become un-stable.
Watch Ted's Holdover video.
To decrease the spin rate would be counterproductive.... like shooting a smooth bore.
Challenging, to say the least.
This I might get:
""velocity peak and the pellet spin peak" occurred at different barrel lengths.... He stated that the velocity peak occurred as much as 8" before the pellet spin rate peak.... "
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I with you Bob. In a standard barrel design, stripping would HAVE to occur and that would be obvious when recovering pellets for analysis. Plus, most AG barrels have a relatively slow twist rate (1 in 16") so stripping would be highly unlikely. Spin is a linear function with twist rate and velocity. I can't see any other possible behavior.
Tom
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In my feeble mind, I can picture the velocity maxing out before max spin. But I picture it as the spin COULD increase after leaving the barrel. Then it would be true that max velocity reached at a different point than max spin. Like I said, I'm no physics major, just looking at it in my feeble minds eye.
:P
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Dave, all projectiles are decelerating once they leave the barrel. Unless they are rocket propelled :) If you had a very short barrel and there was still was some rotational acceleration energy ( like flinging a top with you fingers ) and once the projectile is free of the mechanical restrictions of the rifling - maybe, but I doubt that is the dynamic in a standard barrel design. Maybe this is how a smooth twist achieves it spin with the extremely slow twist rate reported?
-Tom
-Tom
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Some interesting readings...
http://www.accurateshooter.com/technical-articles/calculating-bullet-rpm-spin-rates-stability/ (http://www.accurateshooter.com/technical-articles/calculating-bullet-rpm-spin-rates-stability/)
http://kwk.us/twist.html (http://kwk.us/twist.html)
http://www.czfirearms.us/index.php?topic=14715.0 (http://www.czfirearms.us/index.php?topic=14715.0)
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Yes Dave ...:) The RPM is very fast huh ?
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Yes Dave ...:) The RPM is very fast huh ?
...for what???
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Just my guess, but the only way it would make any sense to me is the initual bast of air sends the pellet down the barrel at a slower rate just before the full charge of air strikes the pellet. Of course this would all take place in milli seconds.
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Yes Dave ...:) The RPM is very fast huh ?
...for what???
I was thinking before you posted the link
It seems like RPMinute is not what we are looking at, it would be RPFoot so 1 in 16" twist would be something
like .75/ft. ? (FPS) X .75 = RPSecond ? ...... Probably doesn't make any sense
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Re-posting the the claim/statement.
the "velocity peak and the pellet spin peak" occurred at different barrel lengths.... He stated that the velocity peak occurred as much as 8" before the pellet spin rate peak....
All taking place in the interior of the barrel. I don't see how it is possible - once the pellet velocity peaks it is all deceleration from that point and spin would have to slow with the projectile due to the constraints of the rifling.
Unless we are talking magic bullets :)
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This is quite interesting but in the end I am sure it is a total time wasting exercise… but isn’t that what hobbies are all about? ::)
The only way I can think of for the pellets angular (rotational) velocity to lag its linear velocity (without shearing the lead) is for the bullet to be elastic, in other words, instead of the bullet following the rifling exactly, the bullet tends to twist around its core, like winding up a watch spring….or like the bullet was made out of rubber. :P
In a “normal” situation where the bullet is not elastic, let’s call it a steel bullet, as the bullet travels down the barrel, it spins exactly like the rifling… there is no lag, and no twisting internal to the bullet.
So if you had a 1 in 12 barrel that was 12” long, you would have one full twist in its length. That is one revolution per foot, or one revolution per barrel length.
Say you had a .22 cal gun that shot 900fps. So at the muzzle vel of 900 fps = 900 rev per sec, or 54,000rpm. But the big deal here is that in a 12” barrel, the linear and angular acceleration are both ridiculously big numbers. The linear acceleration starts at close to 25,000 g’s and drops to about 5,000 at the muzzle, but then (as QV Tom pointed out) drops to zero and then to a negative number as soon as the pellet leaves the muzzle. And the bullet goes from a dead stop, to spinning at a rate of 54,000 rpm within the length of one twist inside the barrel. Personally, I find that truly incredible!!!!! :o
But back to the question….So if the bullet could twist inside itself such that the grooves in the pellet could be lagging on their circumference by… say .003”, how much of a difference in the spin of the bullet would that make? A .003 lag at the circumference is a lag of 0.43%, yes, that is zero point four three percent. So the angular velocity of the bullet (the spin) would be lagging that much, too. Reducing the 54,000 RPM by 0.43% leaves 53,768 rpm. That doesn’t seem like enough of a spin reduction to make a bullet unstable.
I must be missing something, because I don’t see how the spin rate can be much less than what the simple math shows.
But I am very curious to know if the guy really does have an explanation for his claim, because if he is right, that will be a very cool learning opportunity.
Lloyd
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Unfortunately, his posts are hard to understand.... just unsubstantiated claims from "his days at BSA"....
Bob
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That is unfortunate, I thought we might be able to learn something that isn't intuitively obvious.
Thinking about it a little more, the pellet or bullet is really just a screw that you are threading into a tapped hole. It is like a lead (lead as in "lead a parade") screw. Unless something strips out, it stays in sync.
Lloyd
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Unfortunately, his posts are hard to understand.... just unsubstantiated claims from "his days at BSA"....
Bob
See what happens when you wander........... :) :)
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Unfortunately, his posts are hard to understand.... just unsubstantiated claims from "his days at BSA"....
Bob
The rifling literally gears the the rate of spin and speed to be a fixed ratio until the pellet exits the barrel. Then the ratio of spin to speed will increase till the pellet becomes unstable. The frictional drag and frontal area presents far more resistance and speed reduction than the degradation of the rate of spin which will slow down at a much slower rate of change.
Impossible to peak at different places in the barrel. Figure the guy is British and you are separated by a common language.
If I used smileys it would go right here.
1 in 17 or 18 is the rate of twist for most barrels I've worked with. I suspect a quicker spin rate will not be an advantage as there seems little penalty for spinning faster than what is needed for stability and most are already overspinning for that reason.
BSA barrels can be very accurate so maybe from 1 turn in12" to 1in20 it matters little. The Benchmark 2 groove, LW's and HW's are all 1-18 or thereabouts. Rate of twist and number of grooves have always been one of those bones of contention but I have seen all kinds of rifling make good accuracy. I think accuracy has more to do with the reaming and the rifling and now the Taper lapping. I think high end barrel making is an art form and most everyone knows little and the barrel makers who know are not sharing all that much.
All I know is the barrel maker that I use is good as they won the BR World Championship with their first effort. They were responsible for the execution of LD's specifications which were instrumental to the Worlds BR success.
I hope more barrel makers give airgun barrel making a go. It would be nice to have a Stainless Steel .177 ultra match grade barrel. Benchmark/Mac1 2 grooves are .22 only.
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the lenght of the barrel... longer barrel gives the pellet or bullet more time to stabilize in flight . powder burners or pellet guns. i discussed this with my gunsmith . a carbine and a rifle side by side same gun but diffrent barrel lenghts shoot them at a target at a oh say 30 yards and see which gun is easier to keep on target . thats why its harder to hit with a pistol. i got 2 identical rifles out and tried it and well it was that way for me .so im going to go with what my gunsmith said and he has been working on guns and building them for over 40 years . he keeps busy out in the coountyr without advertisements so that says he knows
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............Rate of twist and number of grooves have always been one of those bones of contention but I have seen all kinds of rifling make good accuracy. I think accuracy has more to do with the reaming and the rifling and now the Taper lapping. I think high end barrel making is an art form and most everyone knows little and the barrel makers who know are not sharing all that much......................... Benchmark/Mac1 2 grooves are .22 only.
Tim, your posts always serve to remind me of all that I don't know that I don't know. ;) So I always try and listen and learn. Do you have an opinion on the possible use of a Marlin Micro Groove type of rifling for big bore when shooting lead bullets instead of pellets? It just seems like the lead would displace easier and seal the bore more efficiently. But I really don't have a clue.
Thanks very much,
Lloyd
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Lloyd wrote: " Do you have an opinion on the possible use of a Marlin Micro Groove type of rifling for big bore when shooting lead bullets instead of pellets? It just seems like the lead would displace easier and seal the bore more efficiently. But I really don't have a clue.
Thanks very much,"
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I am convinced that the "perfect" rifling for a big bore air rifle, would do all your are mentioning, and more, while not leading up, would be the early day English Lancaster smooth oval bore rifling, but good luck in finding someone to do such rifling today.
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In my feeble mind, I can picture the velocity maxing out before max spin. But I picture it as the spin COULD increase after leaving the barrel. Then it would be true that max velocity reached at a different point than max spin. Like I said, I'm no physics major, just looking at it in my feeble minds eye.
:P
agree with your assumption. it would stand to reason that spin rate could increase AFTER the pellet left the barrel, due to the lessening of drag, induced by the barrel. however, I would think that the difference would be minimal, at best. ok, now I've got a headache.
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the lenght of the barrel... longer barrel gives the pellet or bullet more time to stabilize in flight . powder burners or pellet guns. i discussed this with my gunsmith . a carbine and a rifle side by side same gun but diffrent barrel lenghts shoot them at a target at a oh say 30 yards and see which gun is easier to keep on target . thats why its harder to hit with a pistol. i got 2 identical rifles out and tried it and well it was that way for me .so im going to go with what my gunsmith said and he has been working on guns and building them for over 40 years . he keeps busy out in the coountyr without advertisements so that says he knows
the longer barrel is easier to keep on target, because of the longer sight radius.
bullet design, ballistic coefficient, velocity and spin rate determine projectile stability. not barrel length.
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Perhaps what he means is that the peak rotational stability occurs a few inches after peak velocity. Not the peak rpm. That would make sense to me.
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............Rate of twist and number of grooves have always been one of those bones of contention but I have seen all kinds of rifling make good accuracy. I think accuracy has more to do with the reaming and the rifling and now the Taper lapping. I think high end barrel making is an art form and most everyone knows little and the barrel makers who know are not sharing all that much......................... Benchmark/Mac1 2 grooves are .22 only.
Tim, your posts always serve to remind me of all that I don't know that I don't know. ;) So I always try and listen and learn. Do you have an opinion on the possible use of a Marlin Micro Groove type of rifling for big bore when shooting lead bullets instead of pellets? It just seems like the lead would displace easier and seal the bore more efficiently. But I really don't have a clue.
Thanks very much,
Lloyd
I don't have any experience with bullets or The grooving you mention. I'm a pellet guy and I think I probably always will be unless they start shooting slugs in BR guns. Not likely.
TimmyMac1
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I'm with you, Bob. Angular and linear velocity should be tied together by the twist rate. You would think any "lag" between them would result in barrel leading or evidence of shearing on the pellet. The barrel would in effect become a file.
My daughter just told me something that surprised the heck out of me. She says things on the internet aren't always true, and people aren't always what they say they are. Who'd have thunk it?
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REALLY?.... I'd never have guessed that!....
Bob
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I'm with Bob. The pellet/bullet is locked into the rifling in the barrel, and as such, it would be impossible for the peak rotational speed to be at a different point in the barrel from the maximum velocity without stripping lead. Also, the rotational speed of the projectile can't increase outside the barrel. Increase in the rotational velocity would constitute an acceleration, which can't occur without energy input.
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Old post, guys.... subject was settled over 2 years ago.... or at least thrashed to death....
Bob
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I'm no expert by any stretch of the imagination, however, it has been my experience that "industry insiders" bearing insight are sometimes more knowledgeable about effects and corrective measure than fundamental understanding of actual cause. Maybe there are other factors at play?
Some things that rattled around my noggin after reading all the responses including the linked websites; 1) a round may encounter inconsistent rifling width making grooves which may be wider than the rifling at some points in the barrel, 2) the air pushing/accelerating the round may cease to provide sufficient force to perpetuate said acceleration before the round leaves the barrel, 3) if acceleration ceases or wanes, drag may result in an unpredictable combination of forces such as shifting of pellet groove side now riding the rifling, stored kinetic energy (possibly manifesting as a torsion or distortion in the pellet), a generated harmonic in the pellet, and 4) interactions in pellet dynamics and barrel harmonics (at any critical point in the barrel?!) may create a set of circumstances that result in an inaccuracy which may be subject to "cure" when the factory changes the barrel length (even if more consistent rifling or change in anything effecting barrel harmonics would have also been an effective cure).
ALL the above is pure conjecture and an overcomplicated way of suggesting the original presentation of insider facts/findings may have been an oversimplification of what may have been 99% effort to fix an issue accompanied by a 1% understanding of the true cause of their problem.
To put it more bluntly, maybe their explanation fits only SOME of the facts. Aww, you guys know what I'm trying to say. Now my head hurts too...
On another forum I got into a discussion about barrel length and accuracy.... A chap who used to work for BSA stated that if the barrel is too short the pellet spin rate would be too low for stability.... I ran the numbers, and of course if you shorten a barrel to the extreme the velocity is so low that the pellet RPM is too low for stability.... that makes sense.... He stated that was NOT the mechanism, however, and that the "velocity peak and the pellet spin peak" occurred at different barrel lengths.... He stated that the velocity peak occurred as much as 8" before the pellet spin rate peak....
I can't see how that is possible.... Surely the pellet, once engaged into the rifling, accelerates both rotationally and horizontally at the same rates.... double the velocity, double the RPM.... To do otherwise would smear the rifling marks on the pellet.... Am I missing something?.... Is there some mechanism that would cause (allow) the pellet to "spin up" at a slower rate than the rifling says that it should?.... We're not talking progressive twist or smooth twist barrels here, BTW.... a standard, constant, helical rifling twist....
Bob
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Two important facts....
1. For the pellet not to travel in lock-step with the twist rate, it would have to strip.... RPM is proportional to velocity, for a given twist rate, period....
2. There is no force available to speed up the rotation of the pellet after it leaves the muzzle.... it can only slow down in both velocity and RPM....
That leaves this statement impossible....
the velocity peak occurred as much as 8" before the pellet spin rate peak....
If the peak velocity occurs at the muzzle, so does the peak spin rate.... If the peak velocity occurs 8" (or some other distance) before the muzzle, and the pellet slows in the barrel before it leaves the muzzle, then so does the pellet spin rate.... I think the gentleman in question came to the wrong conclusion about spin rate vs. velocity, and I see nothing in this thread to change my mind....
Bob
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Bob, You're not missing anything. I agree with You, Tom, Loyd and the gang of others who know that the velocity and spin rate are "in lock-step" as stated here in. I do not agree with the guy Bob encountered whose "... posts are hard to understand.... just unsubstantiated claims from "his days at BSA"....
I think he might have retained a little too much of the "BS" when he left BSA.
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Here is the answer to the puzzle...
The faster you push the handle with the spiral shaft, the faster it spins lol
Hmmmm could be something here :o
https://www.youtube.com/watch?v=c2Yerxer8Zk (https://www.youtube.com/watch?v=c2Yerxer8Zk)
William
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On another forum I got into a discussion about barrel length and accuracy.... A chap who used to work for BSA stated that if the barrel is too short the pellet spin rate would be too low for stability.... I ran the numbers, and of course if you shorten a barrel to the extreme the velocity is so low that the pellet RPM is too low for stability.... that makes sense.... He stated that was NOT the mechanism, however, and that the "velocity peak and the pellet spin peak" occurred at different barrel lengths.... He stated that the velocity peak occurred as much as 8" before the pellet spin rate peak....
I can't see how that is possible.... Surely the pellet, once engaged into the rifling, accelerates both rotationally and horizontally at the same rates.... double the velocity, double the RPM.... To do otherwise would smear the rifling marks on the pellet.... Am I missing something?.... Is there some mechanism that would cause (allow) the pellet to "spin up" at a slower rate than the rifling says that it should?.... We're not talking progressive twist or smooth twist barrels here, BTW.... a standard, constant, helical rifling twist....
Bob
You could teach me for sure, but there is something that happens with BP rifles shooting patched balls.
You can push the ball so fast that it strips the rifleing by slipping the patch. I don't know. Is this what he might be describing, a pellet slipping the rifleing because it is accelerated so quickly that it can not grip?
Obviously this would be a function of depth of rifleing, rate, and breech pressure...
No clue here to be honest.
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i would imagine that the pellet's lack of ability to get up to speed, is what is leaving lead in the barrel as it shears off.
we all know that too long of a barrel, will create too much friction, slowing the pellet down.
at what point does the pellet spin up to speed and then slow back down (fps)? would slowing back down (fps) start to affect the RPM?
if you could run a vid-scope into the bore on a super-long barrel, would you see leading from the bolt to max rpm/velocity and then from there out the barrel?
i suspect the lead-free portion would be inches long, and the max velocity would be found at or during the beginning of this portion.
is this even a viable course thought?
thanks,
rob
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me I do think on a 1:36 .177 barrel an 8" barrel has only turned the pellet about 1/4 of a revolution...and I feel at least a 12 inch barrel is needed...
Just a gut feeling... ;)
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:-X
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If someone can come up with a logical, workable mechanism whereby the pellet can be made to spin faster than the twist rate of the barrel.... or to be spinning slower than the twist rate of the barrel when 8" from the muzzle and then magically catch up.... please post it.... Otherwise, just let this thread die, as it should have 3 years ago.... The answer should be obvious.... If it doesn't make sense, it isn't true.... I regret posting a rhetorical question....
Bob