twist rates


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AFDoc
September 17, 2011, 12:29 AM
Please forgive my ignorance, but I am confused as to what advantages different twist rates afford in a 22lr barrel.

What is the difference between 1 in 9 vs 1 in 16?

Thanks for your time and consideration.

-Travis

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Jim Watson
September 17, 2011, 12:33 AM
The long time standard rifling twist for .22LR is 16 inches. That is ample to stabilize a 40 grain roundnose bullet. I think Lilja even sells a 17" twist.
There is NO application for a 9" twist in a real .22 LR. It is something you put up with if you have a conversion kit for an AR.

ugaarguy
September 17, 2011, 12:36 AM
The faster the twist, the longer (which typically means heavier) the bullet it will stabilize.

1 in 9 is a really fast twist for a .22 LR bbl. I guess if you're shooting only 60gr ammo (like Aguila SSS) then you might want a 1:9 or 1:10 twist. 1:16 is standard for .22 LR. Honestly, I don't think the fairly short range of the 60gr subsonic .22 LR stuff is enough to need a 1:9 twist.

Hope that helps.

rsilvers
September 17, 2011, 12:38 AM
I was excited about the 60 grain, but then I learned it does not reach close to 1000 fps. So I will take a 40 grain at 1000 fps over the 60 grain at a much slower speed.

AFDoc
September 17, 2011, 04:22 AM
Will either twist rate decrease bullet velocity? Will either twist rate improve accuracy? Is there a benefit in accuracy or velocity with either twist rate?

WardenWolf
September 17, 2011, 04:41 AM
I've heard you can overstabilize a bullet resulting in reduced accuracy (I think it causes it to corkscrew somewhat due to the torque).

ugaarguy
September 17, 2011, 05:42 AM
I've heard you can overstabilize a bullet resulting in reduced accuracy (I think it causes it to corkscrew somewhat due to the torque).
You can rip the jacket on thinly jacketed bullets if you run them at too high a velocity through a very fast twist bbl. According to the Speer reloading manual, as long as you keep thinly jacketed bullets under 2800 fps they're fine in a 1:7 twist barrel .223. According to US Army FM 23-9 the M16A2 (1:7 twist bbl) is just as accurate as the M16A1 at 274.2 meters shooting M193 Ball (55gr FMJ) ammo. However, the M16A1 wont stabilize M885 ball (62gr FMJ w/ steel insert). Needless to say, I don't buy the whole over stabilization thing.
Will either twist rate decrease bullet velocity?
Note enough to matter with a .22 LR.
Will either twist rate improve accuracy?
The 1:9 will certainly be more accurate with 60gr ammo, but that's not mainstream .22 LR ammo. I don't know if any .22LR ammo gets even the light bullets moving fast enough to start ripping them in a 1:9 twist bbl. Going back to the 60 gr. ammo, I also don't know of any .22 LR ammo that moves a 60 gr bullet fast enough to get it out to ranges where the increased accuracy of the 1:9 twist will even matter.
Is there a benefit in accuracy or velocity with either twist rate? The 1:16 is going to have less friction. Therefor it will heat less quickly than 1:9, and probably have very slightly higher velocity. The 1:16 twist barrel will probably also last slightly longer than the 1:9, but .22 LR isn't a barrel burner.

The bigger issue, to me, is leading. 1:9 and faster twist in .22 bore (.224 diameter) barrels is used to stabilize longer (heavier), hard jacketed projectiles in .223 & similar calibers. 22 LR barrels are .221 diameter. They're typically 1:16 twist because there are few (if any) .22 LR projectiles that need anything faster to stabilize them. Because .22 LR projectiles are typically plated or plain lead (rather than actually jacketed) they're rather soft. As a result, any bbl. twist faster than what's needed to stabilize .22 LR projectiles is going to lead excessively.

In .223/5.56 I'm a fast twist proponent (based on faster twists allowing the user to shoot the widest range of projectiles). I'll recommend the 1:16 twist for .22 LR to minimize leading, and for the other reasons listed above.

rsilvers
September 17, 2011, 07:02 AM
Twist has no effect on the coefficient of friction.

MistWolf
September 17, 2011, 07:20 AM
You cannot over-stabilize a bullet. It's either stable or it's not. Too many RPMs can affect tractability, but that is only a concern with artillery type trajectories

ugaarguy
September 17, 2011, 07:33 AM
Twist has no effect on the coefficient of friction.
I stand corrected. I mess with this as hobby. I'm serious about it, but still a hobbyist. Mr. Silvers does this for a living from what I've read in his other posts on THR.

rsilvers
September 17, 2011, 07:38 AM
More RPM can increase the dispersion of out of balance bullets. That is about the best reason to not spin much more than needed.

I do own a fast-twist AR in 22lr - which I had made to shoot 60 grain. I have not shot it for accuracy with 40 grain. What I did learn was that I prefer CCI MINMAG 40 grain over the 60 grain just based on cyclic energy which should help reliability.

WNTFW
September 17, 2011, 10:55 AM
This article may be of interest:
http://www.riflebarrels.com/articles/bullets_ballastics/bullet_imbalance_twist.htm

It seems to support what RSlivers said.

I have read some info that overstabilization causes the bullet axis to be off the trajectory path (nose up) cause problems at longer ranges.

rsilvers
September 17, 2011, 10:57 AM
I have read some info that overstabilization causes the bullet axis to be off the trajectory path (nose up) cause problems at longer ranges.

As MistWolf said, that is only true of artillery. It has no measurable effect on rifle bullets.

DrBooom
January 6, 2012, 11:23 PM
Having done a project for a fast twist barrel for 10/22's, I interviewed the various manufacturers of the fast twist barrels, and the Aguila 22 rimfire engineers that developed the heavy weight (60 grain) Aguila SSS round.

Both groups point to the other as the one that specified the 1:9 twist. In talking to Dan Lilja, he said they were asked to produce a fast twist bbl by Rem/Aguila, and just used the closest twist they had on hand, which was 1:9, even though back of the napkin calculations shows a slower twist is likely sufficient.

Lilja now produces a 1:10 twist barrel for 22LR that is a fantastic jewel, and is priced accordingly.

My tech that does the machine rest 22LR testing has a 1:14 twist Anschutz in 22lr that will shoot bug hole groups with 60 grain SS in the summer at his altitude of 7000+'. It's got a 12" barrel IIRC, so the velocity is close to the peak for SSS (SSS peak velocity is ~10", and decreases by ~60-80 fps by 16"), so it's spinning slightly faster than it would from a 16" barrel.

In cold temps, as the air density goes up, the groups open up considerably as the twist is just not quite sufficient.

For normal 40 grain ammunition, even 1:16 twist is overkill, the 1:17 and 1:18 barrels out there make for the benchrest crowd show that slower twists don't hurt accuracy for match 38-40 grain ammo.

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