Barrel twist question

[Redneck19]

The tighter the twist the faster the bullet spins. (right?)
For several years I had heard that 1x7 was more desirable (in AR barrels) because it was "more accurate".

But if this is the case, how come so many precision rifles are 1x10-1x12?

Does the 1x7 twist work better with 16 in. barrels?
I don't think I've ever seen 18-20" barrels in 1x7.

Also, how does bullet weight play into this?

I've read that ARs with 1x7 perform better with heavier rounds (55 gr.+) while 1x9 twist will handle bullet weights down to 45 gr.

(Does the above even make sense? Thanks!)

[bp7178]

1/7 is faster than 1/12. One turn for seven inches compared to one turn for twelve.

1/7 will stabilize just about every round you are going to shoot, 55gr all the way up to 77gr.

I don't know what precision barrels you're looking at. Kreiger makes a 1/6.5 twist, and most (i've seen) are 1/8-1/7.7 etc. Intended use factors in here. Some of the slower twist barrels, the 1/10 and 1/12 are meant for varmint rounds, which are about 40-45gr. Most of the varmint barrels i've seen on the market are all heavier profile stainless barrels.

Bullet wieght doesn't mean anything, per se. It's the length of the bullet, but generally, longer bullets are heavier.

1/7s are pretty much standard, and will work very well with just about everything, with the exception of some of the varmint (40gr-ish) rounds.

The huge exception to all of this are/is handloaders, who will tailer a round to their barrel. I've just gotten into it and haven't fully grasped all the concepts. If you want to know about bullets, chambers and barrels, start reloading. It's a hobby within a hobby.

[Redneck19]

Thanks man!

The precision guns I had seen with slower twist were the FNAR (1x12) and the Springfield M21 (Kreiger, stainless, 1x10) specifically.

[bp7178]

If they shoot different rounds, say a .308, the barrels will have different twist rates.

What works for a .308 isn't the same as a .223...

[Redneck19]

Ohhhhhh... duh!

Thanks for clearing that up.
I guess I thought the principle was the same no matter which caliber was being shot.

[m1ajunkie]

Ohhhhhh... duh!

Thanks for clearing that up.
I guess I thought the principle was the same no matter which caliber was being shot.

From what I've seen, the principle is the same in theory. The actual rates will vary, but I believe the faster the twist, the heavier the bullets.

A faster twist stabilizes heavier bullets. Meaning the 1/7 twist .223 barrel is for the heavier bullets like a 1/10 twist .308 is for the heavier .308 rounds.

[ZRH]

The principal you are thinking of is spin stabilization. It's not an area I'm very familiar with but has to do with conservation of angular momentum, then when you add in air it probably gets more complicated. There are a bunch of different formulas for figuring twist rate for X projectile. This article is pretty interesting read, www.jbmballistics.com/ballistics/bibliography/articles/miller_stability_1.pdf

[ucrt]

I'm probably way off on explaining this but I'll try....


The tighter the twist the faster the bullet spins. (right?)

No, if you are meaning "spin" to be the same as "RPM". Barrel twist and bullet speed determine "spin" or RPM. You can have a bullet out of a 1/7 barrel actually spin slower than a bullet out of a 1/9.



For several years I had heard that 1x7 was more desirable (in AR barrels) because it was "more accurate".

It is "more accurate" only because it stabilizes a wider range of bullet weights. Generally, a heavier bullet resists the wind better than a lighter bullet. In SD/HD, you may want a heavy bullet for penetration, so you need to be able to stabilize it.



But if this is the case, how come so many precision rifles are 1x10-1x12?


Like someone said, different caliber.



Does the 1x7 twist work better with 16 in. barrels?
I don't think I've ever seen 18-20" barrels in 1x7.


Noveske makes an 18" 1/7.



Also, how does bullet weight play into this?

I've read that ARs with 1x7 perform better with heavier rounds (55 gr.+) while 1x9 twist will handle bullet weights down to 45 gr.

(Does the above even make sense? Thanks!)

Heavier bullet weights are longer and not as fast, so it needs a faster twist to stabilize the longer bullet and to compensate the slower spin.

But all of this is "black magic" because these are all just guidelines. I've seen people shoot heavy bullets with 1/9 just fine and ultra light bullets in 1/7 just fine.
But...I have 1/7's.

But maybe it's just me...

.

[ZRH]

But all of this is "black magic" because these are all just guidelines. I've seen people shoot heavy bullets with 1/9 just fine and ultra light bullets in 1/7 just fine.
But...I have 1/7's.

But maybe it's just me...
Flight time makes the difference. Provided you don't disintegrate lightweight projectiles they will "smooth out." Heavier ones are stable for longer with faster twist, they become unstable over a shorter distance with less twist.

[DTHN2LGS]

1/7 in chrome lined barrels or 1/8 in Stainless Steel barrels is what I am using.