Twist rate?

[DCM8513]

This topic was starting to be discussed in another one of my threads but id like to get a little more info. Im going to be purchasing a new M4 profile, 16" AR barrel for my bushmaster as it currently has the H BAR and it weighs WAY too much. Im currently looking at the Green Mt. Barrels as they have the best price that ive seen so far. My question is, what rifling twist rate will best suit me? Ill only be shooting bullets between 55 and 62 grain. Im not looking for target grade precision but i want the best accuracy i can get when moving between these bullet weights. Also, what are your takes on these green mountain barrels. Ive heard their 10/22 barrels are great but nothing for the ARs. Thanks.

 

[Mr. Habib]

I'd go with a 1:9, personally. In my experience, it is fast enough for anything up to about 69gr and still works well for 52 or 55gr.

 

[EvilElmo]

1:7, 1:8, or 1:9 should all work just fine for those bullet weights.

 

[T4rdV4rk]

From what I read, you will not notice a difference between a twist rate of 1:7 and 1:9 with 55 grain bullets. From my experience, my 1:9 bushy shoots better groups with 55 grain than both of my friends' 1:7 barrels. I have not done any comparisons with 62 grain bullets.

I plan to do substantial testing of this over the weekend and get a definitive answer. There are of course other factors besides the barrels that could be affecting this. I'll try to update you after my testing and post some pictures of the guns and groups. Hope this helps.

 

[dom1104]

I own a Green Mountain A1 profile <ultra skinny> 20" 1-12 twist and I love it.

 

[top hat 45]

for that light of a bullit, i recommend the 1/9. les baer customs only use 1/9 and guarentees 1/2 moa accuracy with high end 69gr rounds.

i have a green mtn barrel in one of my 10-22s and its accurate. i kind of figure they wouldnt scrimp on 223 barrels and ruin a good reputation.

i use 1/8 twist in my ar's and shoot 77 gr bullits.

 

[95wrangler]

Good question OP!

 

[42769vette]

1 in 9 will probably be the best, but up to 12 twist should be fine aswell

 

[DCM8513]

From what I read, you will not notice a difference between a twist rate of 1:7 and 1:9 with 55 grain bullets. From my experience, my 1:9 bushy shoots better groups with 55 grain than both of my friends' 1:7 barrels. I have not done any comparisons with 62 grain bullets.

I plan to do substantial testing of this over the weekend and get a definitive answer. There are of course other factors besides the barrels that could be affecting this. I'll try to update you after my testing and post some pictures of the guns and groups. Hope this helps.

Thanks for taking the time. Always nice to have a good excuse to shoot i guess!
And thanks for all the other input, 1:9 it is! I wont be ordering this for a week or two so ill be interested to see if T4rdV4rk has results that say otherwise.

 

[Hop]

It's not bullet weight but bullet length that dictates the required twist rate.

 

[DCM8513]

After looking again at Green Mountain the closets they have to the 1:9 is a 1:8. It sounds like this will be close enough for the price.

 

[palerider0485]

the heaver the bullet, the more twist u need to stabilize the bullet, so the fast 1/7 twist is more for the heavyer grain bullets, shooting the lighter bullets with a fast twist wont hurt anything tho. u cant really stableize a bullet too much, so 1/7 1/8 1/9 will all work fine, u wont notice a difference,

 

[downzero]

1 in 8 is my choice

and I would go with a midlength, not a carbine length (M4) length gas system.

 

[Yeah]

It's not bullet weight but bullet length that dictates the required twist rate.

Getting that on the first page is a milestone accomplishment.

 

[DRob]

Indeed

It's not bullet weight but bullet length that dictates the required twist rate.

This is an often misunderstood aspect of bullet selection. Weight and length are usually directly related until you start shooting rounds like the M855 green tip penetrators. They are 62 gr. but they are very long for their weight. Before I started reloading, the wife was shooting nice little 100 yard groups with her Howa 1500 varminter using 45 gr Win White Box HP and Black Hills 60 gr SP. Both ends of the spectrum for that gun. We ran out of good ammo and happened to have some M855 so she tried it (hey, it's only 2 gr heavier, right?) and couldn't hit a 4'x8' piece of cardboard at 100 yards. Moved to shorter ranges to figure out what was happening and found the M855 was keyholing at 25 yards. Gave away 150 rds of it!

 

[Hop]

This is an often misunderstood aspect of bullet selection. Weight and length are usually directly related until you start shooting rounds like the M855 green tip penetrators. They are 62 gr. but they are very long for their weight. Before I started reloading, the wife was shooting nice little 100 yard groups with her Howa 1500 varminter using 45 gr Win White Box HP and Black Hills 60 gr SP. Both ends of the spectrum for that gun. We ran out of good ammo and happened to have some M855 so she tried it (hey, it's only 2 gr heavier, right?) and couldn't hit a 4'x8' piece of cardboard at 100 yards. Moved to shorter ranges to figure out what was happening and found the M855 was keyholing at 25 yards. Gave away 150 rds of it!

What twist was that Howa?

 

[Yeah]

12"

 

[Yeah]

This is an often misunderstood aspect of bullet selection.

I'd hesitate to name an aspect of bullet selection that isn't often misunderstood. As for the length / weight conflation, a proxy measure is never better than a direct one. Length of any common projectile and all those in hand is directly knowable, directly useable in a simple equation.

 

[palerider0485]

i have a question..... if u shoot 55 grain bullets with a fast 1/7 twist can u over stablize a bullet, i have always been under the impression that as long as your not loading up for super high veloscity u cant have to fast of a trist, so i have always went with a 1/7.

 

[jrainw]

Too tight a twist will lower velocity and raise pressures, only noticeable when you are really pushing the limits though.