Rate of twist

[Buck91]

Sorry, new to the AR and I've been doing a LOT of reading since purchasing my first AR on rate of twist. Seems the general consensus is kind of two sided. Faster rates are recommended for carbine length barrels and faster rate of twist is recommended for heavier bullets. I guess my question is pretty specific. For my new purchase, a 16 inch barrel with a 1 in 7 twist I seem recommendations in the 60 to 80 grain bullet weight (depending on source). Sounds like the 62gr m855 would be pretty ideal. How much of a problem would going to a lighter m193 type round be? Assuming this wouldn't really matter at short and medium ranges? At what range does it become a factor? Or does it?

[ColtSeavers]

The short, short version.

1:7 will shoot everything.

1:8 will shoot everything except possibly the berger VLDs and AMAX 80 grainers.

Bullet construction and shape are very important factors along with barrel twist rate and, as always, every barrel is an individual case study.


If you don't reload 80+ gr bullets, even 1:8 will be just fine.

[wigbones]

I haven't had any issues at short to mid range using 55 grain with my 16" 1/7. I can't comment on long distance shooting with it though.

[Buck91]

Sounds great, thanks for the affirmation!

[lysander]

The short, short version.

1:8 will shoot everything.

1:7 will shoot everything except possibly the berger VLDs and AMAX 80 grainers.

Bullet construction and shape are very important factors along with barrel twist rate and, as always, every barrel is an individual case study.


If you don't reload 80gr bullets, 1:7 will be just fine.

I think you got that backwards:

1:7 will shoot everything.

1:8 will shoot everything except really long bullets.

(According to Bergers the 80 gr VLD should be good in a 1-8 at 2650 fps, but 1-7 is preferred.... the 80gr Long Range BT and longer need 1-7)

The shorter the bullet (generally lighter) the slower the twist you can use, but faster twists shouldn't hurt anything.

[T2C]

It depends on what kind of accuracy you expect. I shot 80g bullets out to 600 yards with a 1:8 twist barrel with success. The standard for my match load in a match barrel was under 2" at 300 yards or I would not use the load/barrel combination in a match. I could meet that goal with a 1:7 barrel with 80g and heavier projectiles. I was never able to meet that goal with 55g - 64g projectiles.

Shooting reduced course matches at 200 yards, my 1:7 carbine did not shoot 50g-64g bullets well enough to shoot a Master score. If under 10" at 300 yards is acceptable, a 1:7 carbine will shoot 55g-64g projectiles at an acceptable level.

Three things I have learned from barrel selection and reloading match ammunition: 1) barrel twist rate is a recommendation for a particular projectile weight, but bearing surface has a lot more to do with accuracy than bullet weight. 2) the barrel manufacturer has a great deal of influence on how well a barrel will shoot. 3) you won't know how it shoots until you actually shoot groups on paper.

The big question is "what kind of accuracy do you expect and at what distances?"

I use ballistics calculators as a guide. I've based barrel and projectile selection on results at known distances. I prefer a 1:8 twist rate for projectile weights of 80g and lighter. A 1:8 twist barrel with a quality 80g projectile will get the job done out to 600 yards.

[ColtSeavers]

I think you got that backwards:

1:7 will shoot everything.

1:8 will shoot everything except really long bullets.

(According to Bergers the 80 gr VLD should be good in a 1-8 at 2650 fps, but 1-7 is preferred.... the 80gr Long Range BT and longer need 1-7)

The shorter the bullet (generally lighter) the slower the twist you can use, but faster twists shouldn't hurt anything.

Absolutely no idea how I got my head that far up into my fourth point of contact, thank you for the course correction.

[MegademiC]

If you are shooting m855 or m193 there is no noticeable difference. As said, even with match grade ammo, you *may* see a difference between 1-7 and 1-8. Iirc most 1/7 twist barrels are actually 1:7.7.

If I was specing a general purpose barrel, I would require it to be 1:7 or 1:8, as long as it's either, other factors become more important, like is the vendor reliable, durability, etc.

Keep in mind, a shorty 1:7 barrel will shoot like crap but a good 1:7 or 1:8 barrel will shoot 77gr or less bullets well (down to at least 50gr vmax).

[T2C]

If you are shooting m855 or m193 there is no noticeable difference. As said, even with match grade ammo, you *may* see a difference between 1-7 and 1-8. Iirc most 1/7 twist barrels are actually 1:7.7.

If I was specing a general purpose barrel, I would require it to be 1:7 or 1:8, as long as it's either, other factors become more important, like is the vendor reliable, durability, etc.

Keep in mind, a shorty 1:7 barrel will shoot like crap but a good 1:7 or 1:8 barrel will shoot 77gr or less bullets well (down to at least 50gr vmax).

What size groups were you able to shoot at 300 yards with a 1:7 twist barrel and 50g V-Max? How did the groups compare to a 1:8 twist barrel?

[Renegade04]