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I've been starting my research into the AR platform lately since I've always wanted one and plan on buying new or building one in the not so distant future. I have been reading up about how the barrel twist affects different bullet weights, accuracy, etc. and I was curious about which twist ratio you all thought was 'the best' or at least the most versatile for the widest range of applications.

Just to eliminate some variables I'm looking at several different types of AR-15s with a 16" barrel that would be mainly used for casual target shooting but also home defense and hell let's throw deer hunting in there just for kicks.

That being said, given a quality AR upper (pick your favorite) which ratio would you recommend for this purpose? Consider that I haven't decided on a specific bullet weight yet for any purpose so feel free to throw out suggestions there as well if you feel it to be necessary to make your case.

I'm gonna go warm up some popcorn now and wait :)
 

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1:7 or 1:8.

1:9 would work as well, though. For deer hunting, just be a little particular about your bullet...I have seen great results with a 1:9 barrel and either the 55gr Barnes TSX or the 52gr TTSX--both of which would be fine choices for defensive purposes as well.
 

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twist rate is all about bullet stability.

three things come into play when choosing:

Bullet weight
Bullet LENGTH
Muzzle velocity

A bullet needs RPM to stabilize, and lots of it. the longer the bullet, the more RPM it takes to stabilize. For example, a lead core 55gr bullet is shorter than a solid copper 55gr bullet and therefore requires less RPM to stabilize.

so how do you get a 55gr solid copper bullet to stablize?

two things:

1. simply increase the rate of twist. simple, no brainer.

2. Increase Muzzle Velocity.

But Professor, how does increasing MV make the bullet more stable in a barrel with the same rate of twist?

Remember, RPM is Revolutions per MINUTE. It's based on time, and twist rate is based on distance. Time over distance = ??? speed! so the more revolutions you can get inside that period of time, the faster it is spinning? right? RIGHT?! there's an easy to use formula to compute RPM. You need to know the rate of twist and your MV.

RPM = MV x 720 / inches-per-revolution

3000 x 720 / 7
3000 x 720 / 8
3000 x 720 / 9
3000 x 720 / 12
3000 x 720 / 14

etc, etc, etc... If you know all that already, then all you need to figure out is the Gyroscopic Stability Factor of your specific recipe. there is a FANTASTIC calculator on Berger Bullet's Website... but you need to know EXACTLY how long your bullet is, the weight, and the MV. That will give you an output in SG. 1.5 is pretty damn close to the bare minimum and anything over that is quite stable. Go below that, the bullet might fall apart at distance.

So what you have to ask yourself is WHAT you need this barrel to do at an absolute MAXIMUM rate of RPM?

This is a question only you can answer.

If you don't want to think about it that freaking hard (and hell i don't blame you, this is mostly useless information that even experienced shooters don't even care about) Just buy a 1:7" and shaddap. :lol:
 

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absolutely! The bullet could spin so quickly that the bullet looses it's structural integrity (you do know that's why they are jacketed, right? :chain: ) and disintegrates shortly after leaving the barrel. Magnum .22's (like the 22-250) have this issue mostly as they are pushing 4000+ FPS so they have much longer twist rates so that bullets stay together.

for really long shots, spin drift will be exaggerated, but most long range shots are done with heavy bullets, not light ones.
 

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So is there a downside shooting a really light/short bullet out of a 1:7 barrel?
From what I understand, the only downside is slightly increased barrel wear. That, and some very lightly constructed bullets may actually come apart from the extreme rotational energy. I have done some reading, and it doesn't happen as often as many internet folks claim it does. Bullets like the Hornady V max won't come apart and can be reliably shot in a 1:7.
 
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absolutely! The bullet could spin so quickly that the bullet looses it's structural integrity (you do know that's why they are jacketed, right? :chain: ) and disintegrates shortly after leaving the barrel. Magnum .22's have this issue mostly as they are pushing 4000+ FPS so they have much longer twist rates so that bullets stay together.

for really long shots, spin drift will be exaggerated, but most long range shots are done with heavy bullets, not light ones.
So would a 50 grain bullet be a problem in a 1:7 barrel? If not, how light would you have to go before you run into problems?
 

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your forgetting something Ryno... MV. I can't answer your question without knowing the MV, and integrety is based on bullet construction...

chances are you wont be pushing the 4000+ FPS line, so i'll just say yes*, with a really big asterisk.
 

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So would a 50 grain bullet be a problem in a 1:7 barrel? If not, how light would you have to go before you run into problems?
It depends on the construction. I think you should be alright with 50 grainers.
 

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I like 1:7, but have a 1:9 also. Depends what you want to do. Check out the page on the ammo oracle and look around here. There are several pages mentioning pros/cons of various twist rates.

Ammo Oracle
 

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your forgetting something Ryno... MV. I can't answer your question without knowing the MV, and integrety is based on bullet construction...

chances are you wont be pushing the 4000+ FPS line, so i'll just say yes*, with a really big asterisk.
I don't think he will be pushing anywhere near 4000 fps with the 16 inch barrel. I'm not trying to be snarky, I just assumed we were talking about a 16 inch barrel, in which case, he won't be near that with 50 grain bullets, or very many bullets for that matter.
 

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This is what I've been told by the old I timers I know (and I'm not saying that its correct, just that its worked for me this long)...

I shoot only 55 and 62 grain regular ammo for minute-of-bad-guy accuracy. I was told a 1:9 is fine for what I'm doing, but no heavier than 62gr. A 1:7 wouldn't necessarily hurt anything, but I wouldn't gain anything either.

Flame on...
 

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I use a 1:8 in my AR. I wouldn't mind having a 1:7, but I can't load 80-90gr VLD's to mag length, so it's not important for me to have a 1:7. 1:8 does fine with 69gr and 77gr and that's fine with me.

For a 16" buy the fastest twist you can find and if you manage to rip apart a bullet in due to the extreme twist at high MV, I'll buy you as many 30 packs as you can lift at once.
 

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I use a 1:8 in my AR. I wouldn't mind having a 1:7, but I can't load 80-90gr VLD's to mag length, so it's not important for me to have a 1:7. 1:8 does fine with 69gr and 77gr and that's fine with me.

For a 16" buy the fastest twist you can find and if you manage to rip apart a bullet in due to the extreme twist at high MV, I'll buy you as many 30 packs as you can lift at once.
My 20" AR has a 1:8 as well. The heaviest I have shot out of it are 77 grain Sierra's. Have you shot 80 grain A-max's before?
 

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I don't think he will be pushing anywhere near 4000 fps with the 16 inch barrel. I'm not trying to be snarky, I just assumed we were talking about a 16 inch barrel, in which case, he won't be near that with 50 grain bullets, or very many bullets for that matter.
absolutely 100% correct, but since there's that possibility of using some weird thin flexible jacket, i give the big asterisk. commonly found copper jacketed lead cored projectiles will have no problems.

So if I had a 30" barrel with a 1:7 twist I could run into issues? So we are back to buy the 1:7 and shaddup......:)
Essentially. I have to reitterate that it takes a lot of RPM to disintegrate a modestly jacketed bullet, but i've done it on the 22-250.

This is what I've been told by the old I timers I know (and I'm not saying that its correct, just that its worked for me this long)...

I shoot only 55 and 62 grain regular ammo for minute-of-bad-guy accuracy. I was told a 1:9 is fine for what I'm doing, but no heavier than 62gr. A 1:7 wouldn't necessarily hurt anything, but I wouldn't gain anything either.

Flame on...
a 1:7 on a carbine gives you a lot of versatility.
 

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absolutely 100% correct, but since there's that possibility of using some weird thin flexible jacket, i give the big asterisk. commonly found copper jacketed lead cored projectiles will have no problems.



Essentially. I have to reitterate that it takes a lot of RPM to disintegrate a modestly jacketed bullet.



a 1:7 on a carbine gives you a lot of versatility.
So on the opposite end of the spectrum, how heavy of a bullet can shoot out of a 16" 1:7 barrel roughly, given we don't know the exact MV?
 
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