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# velocity in relation to barrle twist

#22
02-07-2004, 01:19 PM
 Bronze Member Join Date: Jan 2004 Location: Langholm, Scotland Posts: 96
Re: velocity in relation to barrle twist

Just to make things a little more complicated, for ballistic stability you should be figuring more in RPS(revs per second), than revs per minute..basically because not too many bullets fly for a minute, ha..But I digress..

Velocity does equate in the twist formulae, often because of what was stated previously, bullet jacket failure..Shoot a light jacketed Berger in a fast twist .22 cf, when you see the white 'poof' and 3 out of 5 actually hit the target, you may have stripped the jacket due to the shear forces and high helix..You shouldn't shoot high velocity 40 grain varminters out of 7 twist barrels, ha..

Twist rate depends on a few things..Density of bullet, length of bullet, even shape of bullet to some extent..The higher the velocity, the slower the twist rate required to stabilise the same projectile, yes, but the length of the bullet is the kicker..What you need is to maintain a stability factor of around 1.5, and velocity is indeed a factor in figuring that equation..a rifle shooting a 68 grain 224 bullet at 2800 fps will need faster twist to stabilize than that which hits 4000..

JR
#23
02-07-2004, 01:24 PM
 Bronze Member Join Date: Jan 2004 Location: Langholm, Scotland Posts: 96
Re: velocity in relation to barrle twist

Mikecr,

just one more thing..Greenhill does account for velocity by computing it into the constant it uses to figure twist rate...there are velocity bands which the constant k changes within those bands..

JR
#24
02-07-2004, 01:54 PM
 Platinum Member Join Date: Aug 2003 Location: NC, oceanfront Posts: 3,975
Re: velocity in relation to barrle twist

Yeah I know &lt;&gt;2800fps. Basically irrelevant.

My program utilizes a velocity constant which is more dynamic. However, I'm thinking now that its wrong(after, discussion with a guru over at the BR forum).
His position is that velocity makes no direct difference. Only a secondary effect, which is small enough to dicount for the most part. His software reflects this as well.
I guess I need new software. Those available out there presently are either oversimplified(Greenhill), or overcomplex(those requiring intimate, & internal knowledge of a particular bullet).
Anyway, The Gurus all seem to suggest Revs/distance(RPF)for SG. This is completely different than Revs/time(RPM).
The Revs/ft model discounts velocity (revs/ft/sec) or RPMs as effecting stability.
It uses only revs/ft(twist). Velocity plays a subtle role in effecting the overturning moment. As does air density.
#25
02-07-2004, 05:01 PM
 Bronze Member Join Date: Jan 2004 Location: Langholm, Scotland Posts: 96
Re: velocity in relation to barrle twist

&lt;&lt;Yeah I know &lt;&gt;2800fps. Basically irrelevant.

Ok..ha

&lt;&lt;Anyway, The Gurus all seem to suggest Revs/distance(RPF)for SG. This is completely different than Revs/time(RPM).
The Revs/ft model discounts velocity (revs/ft/sec) or RPMs as effecting stability.
It uses only revs/ft(twist). Velocity plays a subtle role in effecting the overturning moment. As does air density.

hmm..so time plays no factor eh?
#26
02-07-2004, 06:38 PM
 Platinum Member Join Date: Aug 2003 Location: NC, oceanfront Posts: 3,975
Re: velocity in relation to barrle twist

I'm as confused as anyone [img]images/icons/frown.gif[/img]
I really thought RPMs was dominant w/regard to gyroscopic stability. Now it's back to the drawing board for me.

But letting go of RPMs(and the speed which created them) makes some bullets work, that otherwise wouldn't. Like the .224 80gr VLDs, which were blowing up.
With my velocity affected program, at sea level, I could find only 150fps of window that would be marginally stable (SG1.4)and &lt;310Krpm(limit) for the bullet. This at 8TW 3275-3425fps. With a 9TW(my software), velocity would have to be 3750-3850fps. An even tighter window. However, no other program that I can find allows for that 9tw window. Apparently, 9tw will not stabilize that bullet at any velocity.
Anyway, pretending the bullet wouldnt blow up, my program shows SG=1.5 8tw@3475fps. Henry, over @ BR central says that the bullet couldn't be driven fast enough to ever reach an SG of 1.5, even though its 1.4 @3400fps.
I believe Henry knows alot more than I. Also, there are alot of people using that bullet, so there has to be a wider usage window than I can predict(given its RPM limit). Velocity must hold lower value w/regard to stability. Otherwise it would not be a very popular bullet.
Now I gotta fix my stuff. [img]images/icons/tongue.gif[/img]

[ 02-07-2004: Message edited by: Mikecr ]
#27
02-07-2004, 07:25 PM
 Gold Member Join Date: Feb 2003 Location: Texas Posts: 723
Re: velocity in relation to barrle twist

Stability is dependent on foward veolcity because it is the forward velocity that causes the bullet to spin in the first place.

rpm = revolutions per minute (rev/60 seconds)

forward velocity = feet per second (v ft/1 sec)

twist rate = 1 revolution per x inches (1 rev/12 inches)

rpm = forward velocity (ft/sec) * (12 in/1 ft)* twist rate (rev/in) * (60 sec/1 minute)

so

forward velocity = 2800 ft/sec
twist = 1/8 (1 twist in 8 inches)

rpm = 2800 * 12 * (1/8) * 60

rpm = 252,000 rpm

or

forward velocity = 3800 ft/s
twist = 1/11

rpm = 3800 *12 * (1/11) * 60

rpm = 248,727

Almost the same rpm but at much higher forward velocity!

No forward velocity, no twist, no stability.

So if the bullet is going real fast forward you do not need as tight a twist as if it were going only an average forward velocity.
__________________
doug
#28
02-07-2004, 10:31 PM
Re: velocity in relation to barrle twist

dwm
Thats the way I was looking at it too...I think. I realy DON"T have a clue any more though. lol

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