M193 and M855 out of a 1-7" twist barrel

[jmoore]

I don't know why the rumor persists that 55gr bullets won't shoot at all accurately out of a fast twist barrel. Just recently, a fellow at work was all concerned that his proposed build might suffer if he gets a "too fast" twist barrel.

So, here is a quick demonstration of the two standard milsurp types out of a "Government profile" chrome lined 20" Colt barrel of not spectacular accuracy. Since errors tend to compound, I figured this combo would be most apt to produce poor results without the introduction of substandard components.

Ammo:


Good quality and to spec.

Rifle:

"A4ish" build but no A2 stock- was gonna do it, but the SOPMOD's too useful. Colt receiver and parts,mostly. Geisselle FCG (it's USMC approved!).

'80s vintage barrel

TA31RCO-A4 Bad photo, sorry. Actual target used. (And yes, it's patched up from previous range trips.)



10rds M855 off bench at 100yds, no bags, just handheld with a little impromptu support. Getting fancy won't help- this is as expected, and is "good enough"! ~2 1/2 MOA


10rds M193 as above. Zero shift, but not much enlargement. In "match grade" barrels it does do better, but this was a test for gross accuracy loss- I don't see it, do you? Again, about 2 1/2" MOA. As noted above this barrel's not hugely accurate, but it's consistant!

BTW, just for giggles, 18rds Prvi and 2 rds Twin Cities (circa 1970) M193 were then fired standing at 200yds w/ no zero corrections (note that the RH and low shift is also present like on the 100yd target). Given how "off" I've been doing this exercise lately, the results were actually pretty pleasing- the low rounds were all called, only the high hit was a bit unexpected. Too bad you can't get a "skilled" tester!




Oh, now if your lightweight bullets are "substandard" (a recent lot of PMC Bronze 55gr FMJ comes to mind), then the fast twist WILL show the faults more than an "optimized" twist rate- but still no keyholing or other such evilness. Poor bullet jacket concentricity is the real villain here, not a too fast twist.

Anybody else have any actual results along these lines they would like to share?

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[Brian Dick]

Good test JM. I've been shooting 55 grain bullets in 1x7 twist barrels for many years with excellent accuracy. I have an M4 14.5" SBR and a couple of A2 variants with standard 20" barrels. Now, shooting 62 grain and heavier bullets through the old 1x12 twist barrel will be accuracy detrimental. I think lots of folks think that the detriment works both ways but it isn't the case.

[GUTS]

I have the same barrel on my A2 and it handles everything from the M193 to 90grn BTHPs. I also handload 55grn Vmax, 69grn BTHPs, 75grn Amax, and 77grn BTHP's. I have a bunch of 40, 50, and 52grn bullets, I should load some of these and see what happens. I'll let you know.

[jmoore]

Really light varmint bullets often have very thin jackets for maximum terminal performance. If you do a test and seem to be missing the target altogether, the bullets may be bursting in mid-flight due to the higher than designed RPMs induced by the fast twist and high velocity. You may even see a small grey cloud a short distance downrange as the bullet goes "poof", but I've only seen it once or twice!