Is There Something I'm Missing?

[rsterne]

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

[thekid]

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.

[crowpopper]

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

[BenjiHunter]

Considering how fast the pellet accelerates, wouldn't it be difficult for the pellet to reach its full RPM right away?

[Fishbone]

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.... "

[QVTom]

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

[ezman604]

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.
 

[QVTom]

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

[ezman604]

Some interesting readings...

http://www.accurateshooter.com/technical-articles/calculating-bullet-rpm-spin-rates-stability/

http://kwk.us/twist.html

http://www.czfirearms.us/index.php?topic=14715.0

[william71743]

Yes Dave ... The RPM is very fast huh ?

[ezman604]

Yes Dave ... The RPM is very fast huh ?

...for what???

[Bob P]

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.

[william71743]

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

[QVTom]

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

[lloyd-ss]

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. 

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!!!!!

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

[rsterne]

Unfortunately, his posts are hard to understand.... just unsubstantiated claims from "his days at BSA"....

Bob

[lloyd-ss]

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

[QVTom]

Unfortunately, his posts are hard to understand.... just unsubstantiated claims from "his days at BSA"....

Bob

See what happens when you wander...........

[TimmyMac1]

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.

[robert w]

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