velocity in relation to barrle twist

[Mikecr]

Well maybe it isn't.
Greenhill discounts velocity for the most part(which never made sense to me). Now I realize Greenhill consistently calls for tighter than needed twist(based on field results). But is it possible that twist requirements are based on revolutions per foot of travel(which IS different)?
Doesn't take much to derail me!

 

[Michael Eichele]

1-12" WILL NOT stabilize a 68 grain 22 cal bullet. Not even close. A general concensous based on 3 differant calculators came up with about a 1-9" for those long 68 grain bullets. I think (not sure) that the 68 grain bullets are around .925-.950" long. So based upon that at 3500 FPS a 9 twist would be minimum.

Any second opinions are appreciated.

Also, the gyroscopic stabilty factor isnt nessecarily based on RPM's alone, there is much more to it than that, allthough it comes into play. I still think the best way to determine twist is a good twist calculator. What you are looking for is a GSF of 1.5 Above that is not nessecary and below that might hurt the bullets flight in the wrong situation. 1.5 is where everything that can be optimized will be. A 9 twist for your project wouldnt be too fast or too slow. It would be about right for the right ballance of speed and pressure, accuracy and a high BC.

Hope that helps some.

[ 02-04-2004: Message edited by: meichele ]

 

[ackleyguy]

LET ME GET THIS STRAIGHT....THE ROTATIONAL VELOCITY OVER TIME IS WHAT IN FACT DOES STABILIZE THE PROJECTILE?? SOUNDS ABOUT RIGHT

THEN IN FACT DOES TIME = DISTANCE? YES....SO IF YOU IMPART THE PROPER ROTATIONAL VELOCITY TO A PROJECTILE IT SHOULD THEN STABILIZE REGARDLESS OF TWIST? AND AT WHAT POINT DOES ROTATION CAUSE CATASTROPHIC FAILURE OF THE JACKET?WELL I CAN ONLY TELL YOU THIS...A 22-284 WITH A 1-8 TWIST AND 80 GR BERGERS @3680 F/SEC DID , IN FACT BLOW BULLETS TO BITS!!THESE BULLETS WITH J4 JACKETS WERE NEVER INTENDED TO HAVE SUCH RPMS IMPARTED TO THEM...SO IF YOU SLOWED THE TWIST DOWN TO SAY 1-9.5 WOULD IT WORK...I DONT KNOW...BUT I THINK ABOVE 250,000 RPM BULLET JACKETS START TO GET TESTED..SO THERE IS A FINITE LIMIT WITH TWIST AND VELOCITY.....AND THE THROW IN THE BARNES X....FOOD FOR THOUGHT...

 

[Crazycooydog]

meichele

This is getting very tecnical...But thats what I like, as for finding the numbers you request.I not sure how to find them..

This is what I am thinking though:
22-243 middlstead
26"varmiter weight
1-12 twist
hoping to get 3600+ fps with the Hornady 68gr hpbt match bullets
Elevation here is 2500-2800 feet above sea
I mostly shoot in late fall and all winter,temps from +10 deg to -30 deg cel
I think humitity is normaly 70%-95%

Is this enough info to get an idea if this gun/bullet will work.

 

[Crazycooydog]

meichele
Thanks a bunch.I guess it will have to be a 1-9. untill some one says different.

ACKLEYGUY

As for 1-8 blowing up bullets.I know three guys that have 22-284's.
One of them is shooting 68gr hpbt match bullets at 3750 fps and this gun shoots very well.(I know because I have been useing it for about a month now)
#2 guy shoots 55gr gamekings at 4000fps in his and is very happy with it.He had troubles getting the 75gr a-max to shoot,I'm not sure if he tryed the 80gr's or not.
#3 I think he got the 75gr a-max to shoot and used them for awile,this winter they started to tumble and some came apart. He now uses 80hpbt match bullets at 3625 fps and they seem to work fine.

All three of these guns are from the same smith at the same time,the only diff is the #1 is a 30 inch barrle and the other two are 28inch.

How dose one explaine these results???????

 

[Michael Eichele]

The examples given in your last post will drive a man to drink. ALOT!! Have you seen my signature??

Its no joke. I have seen identical rifles side by side that wouldnt shoot the same. Some would shoot other bullets better than some of what the others would shoot. There are things about a barrel that we cant see. Things like molecular structure, steel density, stresses, ect... That list could go on for a while and I am sure that there is more to it than anybody knows. The bottom line is, even though a barrel may be designed to shoot a particular bullet doesnt meen its going to shoot that bullet well. You have to take your chances somtimes. Usually some type of bullet will be found that each indavidual barrel will like.

Barrels are like people. Some like onions, and garlic, some despise oinions and love garlic ect...No 2 barrels are alike.

I have a barrel that was specificly designed and manufactured for the 175 SMK and while I can get a good group with those bullets 50% of the time, it does not excell with them, yet for a barrel made for that bullet, it eats 155, of all types, 168' 178's and 190's with superior results. The 1 bullet it was made for it shoots the worst. But it does not bother me because it shoots so many other really good bullets so well. You will likely find your rifle to do the same. It might just love the bullets youve chosen or it might like everything but that 1. In any event, it will shoot some worthy bullet just the way you want and probably the one you want.

 

[Crazycooydog]

meichele
I here ya. Them 22-284's all were made to shoot the 75a-max with the 80gr match stuff in mind,If all else failed.
I have realy been thinking that it MAY NOT shoot the 68's and I don't really want to limit myself with just that bullet,or even the smaller 60's.All though the JLK's look promising.Realy don't want the hassle of trying to get them across the boarder.The cost of them seems to be affrodable.(really not much more $$ than the hornady's or seirra's)

Thanks for your info. Really apriciated.

 

[JR]

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

 

[JR]

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

 

[Mikecr]

Yeah I know <>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.

 

[JR]

<<Yeah I know <>2800fps. Basically irrelevant.

Ok..ha

<<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?

 

[Mikecr]

I'm as confused as anyone

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 <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.

[ 02-07-2004: Message edited by: Mikecr ]

 

[dwm]

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.

 

[Crazycooydog]

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