1/12 Barrel Question

[twistedcomrade]

My current ammo stash is mostly 55 grn ammo, a mix of IMI 55 grn, Q3131, and PMC XTAC 55 grn. The rest of it is mostly PMC M855 and some 69 grn (IIRC) Federal. I have four AR rifles, three with 1/7 barrels and a SP1 with a 1/12 barrel. My question is, does 55 grn FMJ yeild it's best terminal performace when fired from a 1/12 barrel? I am not trying to start a "which twist rate is better" thread because the answer would probably be 1/7 since it will shoot all bullet weights well. I just want to know which twist rate is better for 55 grn FMJ ammo because my assumption is that 1/12 would be better. Thanks in advance for any info.:)

[Safetyhit]

The 55gr performs very well with the lower twist rate, which is what it was originally designed for. In fact the original M16 used in Vietnam was 1x14. They went to 1x7 for 62gr up through 80gr.

[mkmckinley]

Anecdotally I had a Remington 700P in .223 with a 1:12 twist barrel and it was very accurate with 55 grain handloads. I sold it because for the distances I wanted to shoot 77gr SMKs were more practical. If I'd wanted an accurate 223 for like shorter range varmint hunting or something it would have been great. FWIW it did stabilize hot 62gr M855 but it wasn't that accurate due to the ammo itself.

[twistedcomrade]

Thanks for the replies dudes. I know I am in the vast minority when I say this, but I would love for a manufacturer to offer a 16" midlength barrel that had a 1/12 barrel. It would compliment my SP1 very nicely and since 55 grn is mostly what I have and shoot, ammo would not be an issue.

[mkmckinley]

If you're serious you could have a 1:12" rifle length barrel cut down to 18" and still probably have a gun that would cycle. You could always have the port opened up a little when you gen it cut. 1:12 rifle length barrels aren't impossible to find.

[Safetyhit]

FWIW it did stabilize hot 62gr M855 but it wasn't that accurate due to the ammo itself.

Genuinely curious as to how are you so certain of this. What range are we talking?

In other words it may have appeared to stabilize due to a circular hole, but how can you know that in fact it had not destabilized to enough of an extent as to where it affected the placement? This while taking into account that M855 isn't the most precise round.

[mkmckinley]

Genuinely curious as to how are you so certain of this. What range are we talking?

In other words it may have appeared to stabilize due to a circular hole, but how can you know that in fact it had not destabilized to enough of an extent as to where it affected the placement? This while taking into account that M855 isn't the most precise round.

I guess I don't really understand the question. I got round holes in the target and shitty groups at 100m. To me that means the bullets were flying nose first/weren't tumbling but weren't very precise due to the variation in construction of the round.

[Safetyhit]

I guess I don't really understand the question. I got round holes in the target and shitty groups at 100m. To me that means the bullets were flying nose first/weren't tumbling but weren't very precise due to the variation in construction of the round.


It would have been more decisive to check them at 200+ yards if possible for more discernible traces of wobble, but no big deal. Just don't want anyone to be confused and think that a 1x12 is ideal for M855 or heavier loads in general, however if yours performed well that's fortunate.

[TriviaMonster]

Just run them through a stability calc and you will have your answer. Got an idea on the velocity?

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[mkmckinley]

It would have been more decisive to check them at 200+ yards if possible for more discernible traces of wobble, but no big deal. Just don't want anyone to be confused and think that a 1x12 is ideal for M855 or heavier loads in general, however if yours performed well that's fortunate.

OK I see where you're coming from. I posted my experience with the 62 gr bullet because I was surprised it stabilized (to the extent it did) out of that barrel. I never chono'd it but I'd assume a high MV given the 24" barrel with no gas system shooting NATO pressure ammo. It likely would have been much lower in a sub-rifle length AR. You're right: as velocity dropped at longer distance it may have shit the bed. I didn't mean to imply that M855 would be a good choice for a 1:12" twist barrel.