Any reason not to use a 1/12 twist 16" barrel if limited to M193?

[1168]

My thoughts: Original design (after the 1:14) was for 20" barrel, 1:12. This gives 1 full rotation plus some... They originally used 1:14 just because that's what the .222 parent cartridge used. They went to 1:12 to optimize for 55gr M193 (so says Wikipedia). For a 16" barrel, 1:9 will do about the same as far as number of rotations before the bullet exits the barrel. Lots of people on here smarter than me though....

Revolutions per minute are what matter.

Litz doesn’t seem to think over-stabilization is a problem.

[lysander]

My thoughts: Original design (after the 1:14) was for 20" barrel, 1:12. This gives 1 full rotation plus some... They originally used 1:14 just because that's what the .222 parent cartridge used. They went to 1:12 to optimize for 55gr M193 (so says Wikipedia). For a 16" barrel, 1:9 will do about the same as far as number of rotations before the bullet exits the barrel. Lots of people on here smarter than me though....

The reason the 1-14 twist was dropped for the M16 was stability in cold temperature conditions, if you are limiting yourself to warm to hot temperatures, you're fine.

Back when the .222 was gaining popularity, shorter flat based bullets, in the 45 to 50 gr range were what was available, 1-14 was adequate. The 55 gr boat tail was significantly longer.

[lysander]

Litz doesn’t seem to think over-stabilization is a problem.

Over stabilization is only a problem when shoot at long ranges (near maximum possible) at high launch angles, such as artillery or naval gun fire.

What over stabilization does is fix the orientation of the bullet in space. So, when launched a high angle, say 30 degrees, the bullet long axis does not follow the trajectory, but stays fixed at 30 degrees. And, when it lands at the end of the trajectory, it lands at at 30 degree angle not point first. Needless to say this is bad for armor piercing anti-ship gunfire, and leads to poor fuse operation in artillery.

[DG23]

“Should not” and “curious”. The truth is that 1:7” barrels are every bit as accurate as any other twist, all things being equal. 1:7.7” is quite popular in high-power crowds. Shooters typically want to have “enough” twist to get by and not so much as to cause increased barrel wear with heavier bullets.

If you’re going to shoot a “combat” barrel, what competition shooters do shouldn’t be a major concern when a barrel is “shot-out” after around 2,000 rounds of 80 or 90 gr bullets—meaning it will no longer hold a 6” ten-ring at 600 yards for a ten round string.

This is all mental masturbation and there is no significant difference in the real world of combat rifles.


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Getting 'all things equal' is the hard part for me.

I can easily grab a regular 6920 with the 1/7 twist and take it to the range and shoot side by side comparisons with a 6721 (1/9 twist barrel) but... The 6721 is an HBAR so all things are not really 'equal'.

Can take the same 6721 and compare it side by side to more than a handful of really nice SS, bull barrels with 1/9 twist rates but still again all things are not 'equal' there. Aside of the difference in profiles, there are different chambers in play + the lack of chrome lining in the SS barrels. All of those differences add up...

In my experience, even with m193 for ammo, Pretty much any of my nicer SS bull barrels are noticeably more accurate than any sort of 'combat barrel'.

Both 1/9 twist:



SS barrel is easier to shoot small groups with...

From Left to right - 1/9 CL, HBAR; 1/7 non-CL, med profile; 1/7 CL, gov profile:




With respect to M193 ammo, Last time I took all of the rifles above to the range and shot them all from the same lot / can of ammo - The 6920 on the far right was the worst as far as accuracy for me (accuracy increasing from right to left in my rifles above).

[PracticalRifleman]

Getting 'all things equal' is the hard part for me.

I can easily grab a regular 6920 with the 1/7 twist and take it to the range and shoot side by side comparisons with a 6721 (1/9 twist barrel) but... The 6721 is an HBAR so all things are not really 'equal'.

Can take the same 6721 and compare it side by side to more than a handful of really nice SS, bull barrels with 1/9 twist rates but still again all things are not 'equal' there. Aside of the difference in profiles, there are different chambers in play + the lack of chrome lining in the SS barrels. All of those differences add up...

In my experience, even with m193 for ammo, Pretty much any of my nicer SS bull barrels are noticeably more accurate than any sort of 'combat barrel'.

Both 1/9 twist:



SS barrel is easier to shoot small groups with...

From Left to right - 1/9 CL, HBAR; 1/7 non-CL, med profile; 1/7 CL, gov profile:




With respect to M193 ammo, Last time I took all of the rifles above to the range and shot them all from the same lot / can of ammo - The 6920 on the far right was the worst as far as accuracy for me (accuracy increasing from right to left in my rifles above).

You should expect the heavier contour barrels to shoot better, without a doubt. And most always, unlined barrels shoot better. Same with a stainless match barrel or a CHF barrel.

Ironically, my most accurate barrel is a 1:7” CHF barrel of LW contour; so its kinda of the aggregate to draw conclusions but not the rule.


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[DG23]

You should expect the heavier contour barrels to shoot better, without a doubt. And most always, unlined barrels shoot better. Same with a stainless match barrel or a CHF barrel.

Ironically, my most accurate barrel is a 1:7” CHF barrel of LW contour; so its kinda of the aggregate to draw conclusions but not the rule.


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Was curious about the CHF barrels with respect to accuracy and did pick one up a few years ago but sadly have not managed to get around to trying it out as yet. FN CHF, 20", 1/7 twist, gov profile, rifle length.

If it is even close to what I get from my Colt Comp HBAR rifle I will be more than happy.

(actually bought it as a replacement for the Colt barrel for when that one starts to get long in the tooth...)

[Todd.K]

the slower twist might allow the bullet to yaw after impact and fragment causing more damage than a stabilized bullet passing through leaving a .22 hole.

This is a persistent but completely untrue idea. The spin needed to keep a bullet stabilized in tissue is astronomically higher than a 1/7 barrel.

There is no reason I would choose a 1/12 over a faster twist unless shooting varmint bullets in the 30gr range. If you have a supply of M193 and can only find a 1/12 barrel in stock it’s fine for a plinking/target build.

[AndyLate]

Here is an interesting thread regarding twist rate and yaw: https://www.m4carbine.net/showthread...-to-Bullet-Yaw

Andy

[Colt Carson]

I have read that over-spinning a bullet with an inconsistent jacket thickness can result in decreased accuracy. I also read that a faster twist barrel wears out sooner. I can’t offer proof of either scenario. For me though a 1/8 twist is ideal and I would not choose anything faster unless I planned to shoot something heavier than 77 grain. One thing that could change this though is possibly having to use monolithic bullets due to government/environmental laws. You know, outlaw lead to cripple us. A 1/9 barrel can shoot a 69 grain bullet, but what twist would be needed to stabilize a 69 grain monolithic bullet? What about a 77 grain monolithic bullet? In that case the 1/7 twist might be necessary.

[utahjeepr]

The only guys I know that use 1:12 barrels are bolt action varmint guys throwing 35-45g pills at 4000fps or thereabouts.