Help with twist rate on AR

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kaizer

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Hey guys,

Need some help understanding twist rate. I am planning on building a super accurate (hopefully) AR for some predator hunting. I will probably be going with a 24" heavy barrel, but the twist thing has me going which way and the other. I will probably be using balistic tip ammo, so a little lighter than normal and I can purchase the upper with any one of several twist rates, 1:8, 1:9, 1:12 and so on.

Can anybody help me understand what the best rate would be with this setup?

Thanks.
 
Optimal twist rate can be determined as follows:
d897ff3697a3bbde34972528dbaf3dc4.png

C = constant 180 (150 for velocities < 2800 f/s )
D = bullet diameter (inches)
L = bullet length (inches)
SG = specific gravity (10.9 for lead-core)

It all depends upon the design of the projectile.

Hope this helps :)
 
My AR has a 1:9 twist and shoots 69 grain bullets quite well. So I'd say as long as you aren't going heavier than that get the 1:9. I wouldn't go with the 1:12 twist though as it will limit your bullet choices and just because you only want to shoot light bullets now doesn't mean that's all you'll ever want to shoot.
 
1 in 9 is a good all-around twist for an AR, and is probably the way to go for your application. You should do fine with bullets in the 45-69gr range and maybe a bit beyond(on either end). If you are going to shoot only light, screaming-fast varmint bullets like 40gr Blitz Kings you might want to go with a slower twist like a 1 in 12. If you wanted to do mainly long range work with 77 or 80 grain Match Kings, I'd go with a 1 in 7.5 or 1 in 8. If this is your first AR, I'd definitely stick with 1 in 9 - it is pretty forgiving with a big range of bullet weights.

Good luck,
Erud
 
Thanks for the help. Not my first AR, but my the first one that I want super accuracy out of. I think I will stick with the 1 in 9, should work out well.
 
It's not the weight of the bullet, it's the length. Try using McCoy's McGyro

If you plan using milsup ammo, 1:9 is the best compromise, as it will stabilize M193 and M855 about equally well. Here's FN's dispersion test results.

223_twist.jpg
 
Okay, next questions. Assuming that i can shoot, which I can. And I purchase a high quality upper yada yada yada. Assuming no shooter error, what kind of accuracy can I expect out of a 24 incher with match ammo?

Are we talking .5 or .25 moa? Is that an absolutely crazy number? I want sub-moa accuracy. And I know, there might be a better cartridge/rifle etc. But what is the best I can expect in an ideal situation.
 
I'll go against the grain and say err on the faster side of twists available, even if you're shooting flyweight varmint bullets. I like no slower than a 1:8; you can shoot any bullet that can be seated to magazine depth, and most bullets that can't.
 
I consistently maintain < 0.75 at 100 yards with 50-grain V-Max at 100 yards with my 1 in 9". Took some careful load development, but when I do my part, the rifle always does its part. As stated before, 1 in 9" is pretty tolerant . . .
 
I recently went through the same debate. I settled upon 1:7 twist for my "semi-precision" upper.

Why?

I think one of the barrel makers I talked to summed it up best: "It's no use building your gun around 55gr ammo, because a 1:9 or a 1:7 will shoot it equally innaccurately. It's just crap ammo."

If you're NOT going to be shooting really lightweight varmit rounds (and if you have to ask whether you will be or you won't be, you probably won't be), go with a 1:7 twist. 1:8 is rare (but available), and won't hurt anything but your wallet. 1:9 will also work fine for most stuff (my general-use AR has a 1:9 twist).

Just my opinion,

Mike
 
Assuming no shooter error, what kind of accuracy can I expect out of a 24 incher with match ammo?
Click to expand...

A coworker of mine just got a Colt target model with a 24" SS HBAR. Last Sat we were at the range and he was sighting the scope on the 50yd range. He put five shots almost thru the same hole. After he finished firing the 5 shots, the hole in the paper was no more than 1.5x the width of a .22 bullet. The rifle is a 1in9 twist and he was shooting 75gr Prvi Partizan HP ammo he bought from AIM Surplus. He hand lapped the barrel prior to shooting it for the first time.

In other words, if you find the right combination of barrel and ammo, you can get excellent accuracy with an AR rifle.
 
Absolutely no reason you can't get 1/4 MOA at 100 yards with a well set up gun and ammo tailored to the gun. I'd also agree that if it's a long range gun, specifically if planning on shooting 300-600 yards, I'd go 1:8, but I think 1:9 works fine out to 300 with 55-69 gr. bullets.
 
Spin 'em up! Go ahead with a 1:8 or 1:7 twist barrel. Just be mindful you probably won't want to spin 40-gr pills at full-tilt boogie. But otherwise, you'd be good to go.

My match-gun is set up with a 1-7" twist WOA barrel. It shoots 52gr bullets well at 100 yards. It shoots 77gr bullets really well at 300 yards. It shoot pretty good with 80gr bullets at 600 yards. Luckily, you never have to worry about whether or not the barrel is up to the quality of the ammo.

(The first barrel was a 1-8", which I never fired a 52gr through. And as far as fitting and accuracy of ARs is concerned, well, they're either not too picky or I got lucky. I rebarreled mine in a friend's basement and it shoots pretty well.)

Realistically, I'd be pretty happy with .5-MOA from an AR-15 with a good quality, floated-barrel at 100 yards. Less than that would be stellar IMO.
 
kaizer
Okay, next questions. Assuming that i can shoot, which I can. And I purchase a high quality upper yada yada yada. Assuming no shooter error, what kind of accuracy can I expect out of a 24 incher with match ammo?
Click to expand...

Shooting my RRA Varminter (20-inch, 1/8 twist barrel – with handloaded 77gr Sierra Moly bullets) the best I get for 5 shot groups at 300 yards is 1 inch.
 
As you will tell with the next couple sentences, I know nothing of twist rate. But why is there never a left hand twist? does it matter?

That formula is really neat btw, but what value do you use for 'specific gravity'?

thx
 
I agree, why limit yourself? go with 1/8 and be able to shoot anything you want.1/9 wont let you shoot anything over 70 grains, and 1/12 stops you at about 62 grains. if you have a custom bbl, such as by Douglas, or lothar, or les baer, you can expect half moa, i have a stock cz thatll do it, with two diff brands of factory ammo. these are at 100.
czpics002.jpg
czpics016.jpg
 
It should be noted the formula cited above is a variant of the Greehill formula adjusted for spitzer bullets and for bullets other than lead core. Many ballisticians consider the Greenhill formula outdated, as it was developed for round nose, flat based bullets

The Classic greehill is:

CD^2 / L where C is 150. 180 is used to try and compensate higher velocities in modern cartridges.

McCoy's formula, unsed in McGyro, takes into account the following:O.A.L. of projectile, Nose length, Meplat diameter, Nose shape, Boattail length and End diameter

JBM has a wealth of ballistic calculator. Try this one

http://www.eskimo.com/~jbm/calculations/drag/drag.html
 
I saved this formula that guitar god posted
d897ff3697a3bbde34972528dbaf3dc4.png

After reading his directions, it doesn't make any damn sense. Look at the second portion of the formula, it should be done in this order

SG (specific gravity 10.9 for lead bullets) divided by 10.9 and then the square root of that answer, then multiply that answer by the first part of the formula???

10.9/10.9 is 1, the square root of 1 is 1...

What am I missing here? no I did not do very well at all in high school. Yes I know what the rules are in formulas but I don't get why SG and 10.9 are the same...

thanks guys.
 
Another question on bullet length. Isn't it about surface area contact with barrel length and not overall length?

I mean JRN and a FMJ are going (lets just say they do) have the same exact surface area contact length. Thats the part of the bullet that makes contact with the barrel, why would a tip of a bullet help determin twist rate if it does not make contact?

thanks guys
 
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