1:7 vs. 1:9 twists

[Fang]

I'm hypothetically considering the possibility of looking into the factors influencing a decision regarding the potential purchase of a semi-auto firearm in 5.56. I have the choice between two twist rates for the barrel, 1:9 and 1:7. Everything else is identical.

I plan to mostly shoot 62-grain bullets, with some 55-grainers; whichever barrel I chose would have to shoot those decently. That being said, which twist rate will provide the most versatility? Would I be able to shoot the 40-grain varmint bullets without them getting spun apart out of the barrel if using 1:7? (I understand that the accuracy would suck even if the bullets held together.) How about the converse situation, with heavier bullets and a 1:9 twist?

I've done some research, but also seen some crazy stuff--like people who refuse to go above 55 grains for a 1:9 twist barrel, which seems a little bit off to me. Does anyone have some actual experience they'd like to share?

[Clinotus]

Hypothetically I applaud your research efforts should this mental exercise bear fruition.:wink:

1:7 heavier and longer bullets.
1:9 lighter weighted bullets.

Most ARs I have are 1:9 as it gives me more options for what I can shoot. I wrote a piece on this awhile back...let me see if I can find it.

[Cyrano 4747]

I did a bunch of research on this before I got mine, and I did a lot of testing with a buddy's 1:9 M4gery to see if the ARFCOM groupthink held water. Surprisingly enough, it did. IF you poke your head over there you'll find some good explanations of ideal barrel/bullet combos.

The one thing to remember, though, is that barrel length also affects it. Without going into too much detail, here are the outlines of what I remember from when I put together my 5.56 rifle:

1:9 14.5" barrel - good for 55gn, will shoot 62 but not as accurately (still not too bad, battle-rifle accurate), about the same bleed off in accuracy if you go for the super-light varmint stuff

1:7 14.5" barrel - good for 62gn, can do heavier but really optimized for 62. 55 works, but not as accurate (although not terrible). Lighter stuff tends to either shoot really poorly or fly apart.

1:9 20" barrel - really good for 55gn, I've read reports that it's ideal for slightly lighter loads in the ~50 ballpark. Can do lighter stuff, less accurately, but if you get down to the super-light stuff accuracy gets really marginal. Good up to about 62gn same as the 14.5" but past that accuracy goes right in the shitter as well.

1:7 20" - good for 62gn, optimal (in my experience) seems to be right around 69gn, although I am fairly certain you could develop a really good 62gn load for it too. Heavier bullets seem to be no problem, as long as it can fit through the magwell. The upper limit on this seems to be about 75gn - past that and you're in single-shot bolt action territory. 55gn shoots acceptably, but not great. For a real-world comparison, I get about the same accuracy out of 55gn handloads as I get out of 62gn Wolf. The super-light stuff overstabalizes to the point of uselessness. Even if you manage to have it not fly apart it's not going to be going straight enough to hit that groundhog you're trying to explode at 100 yards.

edit: just realized this probably wasn't clear: longer barrels seem to favor slightly heavier bullets all around. 62gn seems to be the sweet spot for most 1:7 14.5" barrels, while 69 or 72gn seems to be ideal for the 20" barrels.