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.
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.
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
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.
What is crappy about it? What does it do poorly?
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What’s the length difference between the M856’s projectile and more practical ones, such as 70 TSX?
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.
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.
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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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.
This. Its been proven. Molon shot 52gr SMKs out of 1/9 and 1/7 barrels with the same results.
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.
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.
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…