AR experts: M193 O.K. in 1:7 twist barrel?

[dodging230grainers]

Just bought a NIB Colt 6920

With its 1:7 twist, in your experience, will standard M193 55 grain FMJ stabilize well enough to print the groups this gun was made to shoot?

Will it vary by barrel, or is it safe to say I can order a couple cases of M193 without worry? I'd prefer to stick with M193 as its bother cheaper to shoot/stockpile, and serves as my HD load.


thanks!

 

[TexasRifleman]

Overspinning the bullet seems to have little impact on accuracy but it does cost you some velocity.

Although that barrel was optimized for SS109, it will shoot the 55gr just fine. You will experience slightly more barrel wear shooting nothing but the lighter bullet weights but we're talking thousands of rounds before any difference would show up.

In the perfect benchrest sterile environment the best accuacy will come with the slowest possible twist that will stabilize the bullet, but it's hardly noticeable in the case you're talking about.

99% of the shooters out there won't notice the difference.

Now, you try some 40gr bullets in that barrel you may begin to see some problems. The bullet may not stay together. But down to 55gr the 1/7 should be fine.

 

[dodging230grainers]

Thanks Tex.

Never knew it could cost me velocity. Hopefully not any meaningful amount.

 

[TexasRifleman]

Hopefully not any meaningful amount.

Not that you'd notice for most distances you'd use the 55gr but I saw a tiny bit less when I played around with it a few years ago.

 

[arthurcw]

You will experience slightly more barrel wear shooting nothing but the lighter bullet weights but we're talking thousands of rounds before any difference would show up.

I thought the 1/7 would wear faster no matter what. So running lighter bullets in the 1/7 is what wears it faster? so nu?

 

[TexasRifleman]

I thought the 1/7 would wear faster no matter what. So running lighter bullets in the 1/7 is what wears it faster?

Dunno for sure, certainly never done any testing but everything I've read says that. Guess I should have said "I've always heard that".

 

[arthurcw]

I used to think to myself that when I died I was going to ask God all sorts of meaningful esoteric questions in my exit interview. Now all I want to know is why he made barrel twists such a complex thing.

I mean, come on Big Guy. Gun, Bullet, Bad guy… I just want Gun “A” to send Bullet “B” to Bad Guy / Zombie “C”. Why you gotta make-a dis so complified!

 

[TexasRifleman]

Why you gotta make-a dis so complified!

And the rules are clearly fuzzy. I have a 11.5" 1/9 twist barrel on an AR. I shoot the 75gr Hornady TAP ammo in it. It's spot on accurate and has no keyholing at all.

According to the "rules" those bullet should be tumbling like mad.

 

[arthurcw]

According to the "rules" those bullet should be tumbling like mad.

I seriously think very mean and sadistic people make up these rules to drive anal retentive people like me mad when trying to decide on a firearm.

Just my luck, I'll get a 1/7 that will send 75 grn TAPs end over end and stabilize Varmint Rounds.

All you nice High Roaders in Texas will come visit me on the funny farm the day that happens. Right?

 

[dodging230grainers]

I'm gonna guess it does vary from bbl to bbl

Two cases of Winchester Q3131, here I come!

*prays*

 

[Float Pilot]

The M-193, 55 grain bullets (19.3 mm long projectile) work just fine in the M-4s and M-16A2 with the 1 in 7 twist. During state-side training we were told to use up the old ammo form training and rifle matchs. There was not any big difference in the barrel wear. We fired thousands of rounds.

The M-196 tracer projectile (red Tip) is 23.1 mm long and works great in the 1 in 7 barrels. But it sure makes them dirty fast. My unit had to shoot up a whole conex full. It caused nothing but lots of extra cleaning.


The 1 in 7 twist was not developed for the M-855 62 grain projectiles. During testing it was found that a 1 in 9 twist worked just fine for that bullet.

HOWEVER::: The M-856 tracer round needed a tighter twist. Particularly in the SAW (M-249). So the 1 in 7 twist came about.

The M-855 ammunition, (green tip) frequently called SS-109 by non-military types, has a 62-grain, gilded metal-jacketed, lead alloy core bullet with a steel penetratorprojectile that is 23mm long.

The M-856 Tracer has a 29.3 mm long projectile. So you can see why the heavy twist rate was needed. It has a orange colored tip.

 

[Gary G23]

All four of my carbines are 1:7. I shoot M193 99% of the time. No issues.