Too fast of a Twist?

Roskoe · 06-08-2005, 12:29 PM

I was helping out at law enforcment firearms school yesterday and an instructor with some fairly impressive credentials was talking about the evolution of barrel twist rates in Colt AR-15 rifles. He said more than once that the 1:7 twist barrels were designed for bullets in the 62 to 80 grain weight class, and would not stabilize the 55 grain and lighter projectiles. Has anyone ever heard of a very fast twist causing a bullet to become unstable? I sure haven't. Roskoe

charlie brown · 06-08-2005, 12:48 PM

I have heard that theory before. Not sure if it is true or not. I could see some problems maybe. Say a .22-250 with a 1:7 twist, and shooting a 35 grain Berger at well over 4000 FPS, I could see that causing the thin jacket to have some problems before it gets to the end of the bore. That is a lot of force onto a small projectile. Thats the only thing I could think of, actually causing damage to the bullet, which might cause it to be unstable???

johnch · 06-08-2005, 04:42 PM

I have several targets that look shotguned with small parts of jacket and lead from my 1-7 twist barrel AR 15 .
The bullets were Hornady SX , several brands of Hornet class bullets ect ,thin jacketed varmit bullets.
The bullets come apart in mid flight.

I have great luck with bullets as light as 50 gr in my 1-7 barrel with groups.

For chucks I shoot a 55 gr Balistic tip out of my barrel. ( can't remember who made it , NOT Colt junk)

Johnch

biscuit jake · 06-08-2005, 07:06 PM

As I recall, we were able to fire the old, light 5.56 Nato bullet in the late model M16s with the fast twist. In fact I think we had to use up old stocks during annual qual. Senility is a terrible thing... I think the bullet was 55 grain FMJ before we went to the 62's.

They wouldn't let us shoot hollow points or the SX bullets, but I am confident in johnch's results.

Now, if anyone knows if a 1 in 12" twist will stabilize a 60 grain bullet, I would like to know.

Roskoe · 06-08-2005, 08:29 PM

I used to have a custom AR-15 with a Shilen 1:12 that would stabilize the old 60 grain Nosler solid base. I think it was about at its upper limit, though. They say the 62 grain NATO SS-109 is marginally stable out of a 1:10 - solidly so with a 1:9.

Called Sierra Ballistics this afternoon. They said "Hogwash" at the notion that a bullet could be unstable with too fast of a twist. They did, however, indicated that a bullet with way more twist than it needs doesn't fly well at real long range. I guess it tends to continue through the air at the angle it left the bore - and when its dropping at a 15 degree angle at long range, its still flying through the air with its nose facing a little uphill - can make an oblong hole in the target, even if it is completely stable.

vangunsmith · 06-08-2005, 10:06 PM

there has been a lot written on 22 cal barrel twists since the advent of the Militarys AR-15. when the first .224 cal cartridges came to be they were found that a 1-14 twist was the best according to barrel makers Blilss Titus and the Pfeifer rifle co.And i can say that a 1-12 twist will stablize a 60 gr. bullet. I shoot 70gr.speers and barnes in mine all the time,and they shoot just as accurate as do the 50gr. speer tnts and 55 gr bullets. I have found that most of the time these instructors in law enforcement,and as the military i think read too much of there own press or some thoughts of some whom don,t know and pass it on as written in granite! I have been down this road with other people whom have blinders on there brain before. And if they don,t believe me i will dare them to sit out there and be my target. Load them up and give the 60grainers a try.I,m not saying for sure they will for all barrels are different but they should. vangunsmith

HighDesertWolf · 06-09-2005, 04:43 AM

Ive heard before particularly about thin jacketed varmit bullets, if the rotations of the bullet upon leaving the barrel are to high for the structural integrity of the jacket to handle, the centrifugal force will peel the jackets away from the lead core. I would imagine this would cause the bullet to lose stability.

Slamfire · 06-09-2005, 08:20 PM

Before WWII it was difficult to make well balanced bullets as small as .25 caliber. That's why Remington standardized the .257 Roberts with a round nosed bullet. A lot of things that were true then became set in concrete in some shootier's minds. Nowadays it is difficult to overstablilze a bullet.

driftrider · 06-09-2005, 09:19 PM

The only really problem with shooting a light for caliber bullet out of a fast for caliber twist is the possibility of making the bullet self destruct enroute to the target. In order to stabilize a bullet of a certain diameter and length, you need to make the bullet spin at or above a certain RPM. This is why a typical .22-250 has a 1:14" twist, while a typical .223 has a 1:12". Shooting the same length bullets at higher velocity requires less rifling pitch to achieve the same desired RPM.

Of course, as bullets get heavier they get longer, which means they must spin faster to be stable. Additionally, for a given cartridge, heavier bullets mean less velocity, which makes even more rifling pitch necessary to achieve the minimum stable rotational speed. The caveat to increasing the rifling pitch to stabilize very long bullets at slower velocities is that when you propel a standard to light for caliber bullet to much higher velocities, the rate of rotation becomes abnormally high. For a hearty bullet like a 55gr NATO ball FMJ, such rotational rates don't effect it at all, but with a thin jacketed varmint bullet designed to disintegrate impact the excessive spin can cause the bullet to expand and fragment in mid-air.

The result is usually a cool grey smoke trail and fragmentation hits on the target.

Mike

Roskoe · 06-10-2005, 05:36 PM

I haven't, so far, had the gray smoke trail problem in a .223 - but I have sure seen it in the .22-250 and .220 Swift. One thing you imply in your post, Mike, is that a .223 with a 12" twist might provide similar stability as a .22-250 with a 14" twist? Because of the RPM similarity? While you can make up for twist by increasing the speed - speed doesn't help anywhere near as much as it one would think.

This became very apparent a few years ago when I was playing around with a caliber called the .300 Whisper. It shoots .308 rifle bullets at just below the speed of sound. I was almost shocked to find a 200 grain Sierra Matchking was completly stable and accurate out of 1:10 twist at these speeds. A 1:8 twist was good for the 240 Sierra Matchking. Not much different than much bigger calibers driving these bullets almost three times as fast.