View Full Version : Any reason not to use a 1/12 twist 16" barrel if limited to M193?
If I built an AR that would be fed EXCLUSIVELY with M193 is there any reason I should NOT use a 1/12 twist 16" barrel?
Is there any benefit to doing so, such as possible increase in velocity or terminal effect?
Thanks
TED
There's no practical difference in velocity or terminal performance between barrels with 1/7 or 1/12 twists.
Rounds fired from a 1/12 twist barrel start off at a lower rpm and start losing stability at a shorter range. I don't know at what yardage that would be or if it makes any practical difference.
Colt Carson
01-15-21, 22:39
If I built an AR that would be fed EXCLUSIVELY with M193 is there any reason I should NOT use a 1/12 twist 16" barrel?
Is there any benefit to doing so, such as possible increase in velocity or terminal effect?
Thanks
TED
Why paint yourself into a corner? However, if you plan to use FMJ ammo for self defense, 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. But no, in today’s world I would never want a 1/12 twist.
There's no practical difference in velocity or terminal performance between barrels with 1/7 or 1/12 twists.
Rounds fired from a 1/12 twist barrel start off at a lower rpm and start losing stability at a shorter range. I don't know at what yardage that would be or if it makes any practical difference.
A regular child's top spun by hand and spinning on a table top, takes about 5 to 10 seconds before it shows signs of instability. So accounting for the difference in angular moment of inertia, angular speeds and drag from the table, the average bullet will probably remain stable for 5 to 6 seconds when spun at 180,000-200,000 rpm.
So, how far will a bullet go in 5 to 6 seconds?
A regular child's top spun by hand and spinning on a table top, takes about 5 to 10 seconds before it shows signs of instability. So accounting for the difference in angular moment of inertia, angular speeds and drag from the table, the average bullet will probably remain stable for 5 to 6 seconds when spun at 180,000-200,000 rpm.
So, how far will a bullet go in 5 to 6 seconds?
Oh dang
PracticalRifleman
01-16-21, 08:14
https://uploads.tapatalk-cdn.com/20210116/7f0a74cc864805dbc2eb06f942048cb7.jpg
It will be stable. I think the better question is “why go with a 1:12” twist?”
The answer, I can’t think of a reason.
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Only reason I can think of is if you are going to be shooting a lot of .22 rimfire, the 1/12 is closer the the .22's 1/16 so should in theory give better accuracy.
It will perform tolerably with 193, probably gonna see more drift as range steps out.
Can you explain why you would choose to limit your ammo selection and settle for "meh" from your rifle?
You guys do realize the 1/7 is a pretty crappy twist rate that was only picked to stabilize super long M856 tracer in extreme low temperature. Kind of a shame it became the default for civilians instead of something like 1/8 or 1/8.5.
PracticalRifleman
01-16-21, 11:15
You guys do realize the 1/7 is a pretty crappy twist rate that was only picked to stabilize super long M856 tracer in extreme low temperature. Kind of a shame it became the default for civilians instead of something like 1/8 or 1/8.5.
What is crappy about it? What does it do poorly?
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You guys do realize the 1/7 is a pretty crappy twist rate that was only picked to stabilize super long M856 tracer in extreme low temperature. Kind of a shame it became the default for civilians instead of something like 1/8 or 1/8.5.
What’s the length difference between the M856’s projectile and more practical ones, such as 70 TSX?
Glock9mm1990
01-16-21, 11:32
If I built an AR that would be fed EXCLUSIVELY with M193 is there any reason I should NOT use a 1/12 twist 16" barrel?
Is there any benefit to doing so, such as possible increase in velocity or terminal effect?
Thanks
TED
I don’t know why you would limit yourself but if you are dead set on only M193 I would just get a 1/7 20” barrel from a reputable manufacturer.
What’s the length difference between the M856’s projectile and more practical ones, such as 70 TSX?
Don't know but here is a picture of one with some other 62 grain bullets. The reason it is so long is the requirement to trace out to 800 meters.
Found a pic next to a 70 TSX.
64898
What is crappy about it? What does it do poorly?
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Should not be as accurate, I believe most match shooters use 1/8 but that is not a apple to apple comparison, I don't know if anybody makes a combat grade chrome lined 1/8 that could be used to confirm the difference. Don't get me wrong, some 1/7 barrels shoot quite well, just curious if 1/8 would be noticeably better.
PracticalRifleman
01-16-21, 12:30
Should not be as accurate, I believe most match shooters use 1/8 but that is not a apple to apple comparison, I don't know if anybody makes a combat grade chrome lined 1/8 that could be used to confirm the difference. Don't get me wrong, some 1/7 barrels shoot quite well, just curious if 1/8 would be noticeably better.
“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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vicious_cb
01-16-21, 12:44
You guys do realize the 1/7 is a pretty crappy twist rate that was only picked to stabilize super long M856 tracer in extreme low temperature. Kind of a shame it became the default for civilians instead of something like 1/8 or 1/8.5.
Because its not. There is no downside in shooting light bullets out of a 1/7 and is just as accurate as slower twists given equal barrel quality.
An issue only arises when you shoot really light thin jacketed bullets like 36gr varmint grenade out of 1/7 20" barrel. The same barrel will shoot 40gr vmax just fine.
“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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This. Its been proven. Molon shot 52gr SMKs out of 1/9 and 1/7 barrels with the same results.
Because its not. There is no downside in shooting light bullets out of a 1/7 and is just as accurate as slower twists given equal barrel quality.
The an issue only arises when you shoot really light thin jacketed bullets like 36gr varmint grenade out of 1/7 20" barrel. The same barrel will shoot 40gr vmax just fine.
This
My 18" CL CHF 1/7 puts 50 gr. tipped bullets into satifyingly small groups.
Andy
Thanks. My point is that, while the tracer projectile is longer, it’s not as much longer as you might expect.
Don't know but here is a picture of one with some other 62 grain bullets. The reason it is so long is the requirement to trace out to 800 meters.
Found a pic next to a 70 TSX.
64898
If I recall, the DSM, at least Compass Lake SAM-Rs were 1/7.7” & Douglas DMR 1/8” instead of 1/7, supposedly for accuracy, but I don’t know how practical the differences are vs 1/7. As noted, Molon showed no difference using a typical (for him) methodical approach & he’s a wealth of unbiased info.
Won’t a faster twist theoretically heat up quicker than slower? Again, I doubt it’s significant, but it seems there are members here way more mathematically savvy than I am.
To answer OP with my own experience: I had a .223 Hbar AR custom made in the late 1990s with a 1/9 twist, because that’s what the internet & ‘smith stated was “best...” a few years later, when the 77grain OTM became common, I was maxed at 69gr, because it was all the 1/9 would stabilize. Within 300 yards I was shooting, it didn’t matter, but I did eventually sell that gun & had a Krieger 1/8 built for 77grain OTM use.
If it were me, I would go with a 1/7 or 1/8, because with the way the ammo situation is currently, who knows what 5 years will bring. Or, if restrictions on replacement parts is in the future with Biden & Democrats in power. But if I had 10K rounds of M193 in my hands, I wouldn’t think twice about using a 1/12....but I would still buy a 1/7 barrel....but I like spares & options, and have had spare screws evolve into retro carbines & rifles....
Best of luck with your build & choices, OP. Options: keep them as open as possible.
If I built an AR that would be fed EXCLUSIVELY with M193 is there any reason I should NOT use a 1/12 twist 16" barrel?
Is there any benefit to doing so, such as possible increase in velocity or terminal effect?
Thanks
TED
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....
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.
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.
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.
“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:
https://i.imgur.com/Zch7NUV.jpg
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:
https://i.imgur.com/Qdt2BDf.jpg
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
01-17-21, 09:57
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:
https://i.imgur.com/Zch7NUV.jpg
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:
https://i.imgur.com/Qdt2BDf.jpg
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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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...)
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.
Here is an interesting thread regarding twist rate and yaw: https://www.m4carbine.net/showthread.php?35029-Barrel-Twist-in-relation-to-Bullet-Yaw
Andy
Colt Carson
01-17-21, 17:35
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.
The only guys I know that use 1:12 barrels are bolt action varmint guys throwing 35-45g pills at 4000fps or thereabouts.
If I built an AR that would be fed EXCLUSIVELY with M193 is there any reason I should NOT use a 1/12 twist 16" barrel?
Is there any benefit to doing so, such as possible increase in velocity or terminal effect?
I don't think it makes much difference in practical terms.
In a 14.5-inch twist M16A1 carbine barrel you still got at least one full twist before the round leaves the muzzle. I've shot tens of thousands of M193 and commercial 55-grain rounds out of 1-7 twist 14.5 carbines as well as the old 1-12 twist M16A1 carbine.
I shot out at least one 1-9 twist carbine (I don't remember the brand) with M193 and commercial 55s.
A regular child's top spun by hand and spinning on a table top, takes about 5 to 10 seconds before it shows signs of instability. So accounting for the difference in angular moment of inertia, angular speeds and drag from the table, the average bullet will probably remain stable for 5 to 6 seconds when spun at 180,000-200,000 rpm.
So, how far will a bullet go in 5 to 6 seconds?
Was it an African Swallow?
M16A1 shooting 55s, somewhere between 1500 and 1700 yards.
64917
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