AR twist rates

[ArmyBrat61]

I have been pondering delving into the AR world and with my research, I have come across different twist rates in different length barrels. My question involves twist rate and bullet grains. Does a 7:1 twist need a different/heavier/lighter projectile than a 9:1 twist? Are there advantages to the slower 7:1? Is the 9:1 more desirable? Any help is always appreciated.

[z0mbies fear me]

The slower twists typically do better with lighter bullets at least in a 223. I have one with a 1in 12" twist that shoots the light stuff great (40gr and 50gr) but sucks with anything over the 55 grainers. I also ordered some 30gr and 35gr to try out in it.

[z0mbies fear me]

If you are going to be shooting primarily 55gr and 62gr I'd go with a 1 in 9" if you plan on primarily shooting heavier/longer bullets 77gr, tracers, or lead free(long) the 1 in 7" would probably serve you better.

Found this video after posting my answer, I believe it explains the differences pretty well, and it's just 3 and a half minutes long.

Tech Tip: AR-15 Twist Rates: https://youtu.be/oiXn-1jirWs Edited July 7, 2015 by z0mbies fear me

[Ted S.]

I built two bare bones AR-15 with a 1:7 twist in each. Both of the people I built them for plan to use 55gr, but I recommended at least 62gr with that twist rate.

 

I have a 1:8 and shoot 62gr green tips. I

[whiskey]

I'm more visual...

[ArmyBrat61]

WOW!  So much info...thanks guys...Looks like I'll be getting a Stag upper with a 9:1....mostly shooting the 55gr as it seems plentiful around here.....so generally speaking, the higher the twist rate the lighter the bullet for best performance....  Now to get my other parts and accessories. This sure is different than building an AK.

[Dolomite_supafly]

I have found that the heavier the bullet for a given twist the more accurate it will be. That is a generalization as some barrels are different on an individual basis. The best weight for my 9 twists has been 68 or 69 grains. I have also found that 7 twist is less accurate than 9 twist with 55 or 62 grain bullets. But as far as a self defense gun the practical accuracy will be about the same. A 2" group and a 3" group at 100 yards is still going to result in a hit to at least 300 yards.

 

My 12 twist gun shoots M855 pretty close to MOA and is stable to at least 100 yards. You can see some slight feathering of one edge of the bullet hole indicating the bullet is slightly off kilter but that is exactly what I want. With it like that it will tumble immediately upon entry into an organic, moving target causing as much damage as possible as it hopefully fragments. With faster the twist the deeper the bullet will go before it tumbles and begins fragmenting. Where a 12 twist bullet might go 2" before tumbling a 9 twist might go 6" and 7 twist might go 12". Those numbers are generalizations but that is the idea. But if you increase the bullet weight, or length, for a given twist it will reduce the distance before tumbling. Like if the 9 twist goes 6" with a 62 grain bullet it might only go 2" with a 77 grain or 7" with a 55 grain bullet. So first you need to figure out the range of bullet weights you are likely to be shooting then pick a twist to match it rather than pick a twist and try to make it work.

 

For 55 or 62 grain bullets, which are the most common, I would pick either a 9 or 12 twist barrel. And with a 9 twist you can easily shoot up to 73 grain bullets and most 9 twist barrels will shoot 77 grain bullets that are stable to at least 100 yards. The only reason I would pick an 8 twist or 7 twist over 9 twist is if you are shooting HEAVY bullets. An 8 twist will easily stabilize anything that will feed from an AR magazine so there is no need for a 7 twist unless you are shooting bullets so heavy they have to be loaded single shot. Something else about 7 twist is you can sling the jackets off of bullets from centrifugal force. But if you MUST have, or already have, a 7 twist barrel I would load it with the heaviest bullets I could find for self defense because the heavier, and longer, the bullet for a given twist the faster it begins the tumbling and fragmentation cannot happen unless the bullet tumbles.

 

With FMJ bullets there are several methods of wounding. One is just going through the target, another is the bullet tumbling and finally the bullet fragmenting. A bullet that is too stable can, and will, pass through the target before it begins to tumble. We are seeing this in Afghanistan with the thin bodies, faster twists and light (for 7 twist) bullets. For a  bullet to fragment it must have be two things, unstable enough to tumble first and fast enough to cause the bullet's integrity to fail causing it to fragment. If the bullet is too stable it will pass through before tumbling creating a 22lr like wound but if the bullet is unstable it will tumble and if the velocity is fast enough the bullet will fragment. And twist rates along with bullet weight/length determine where the bullet tumbles. As I said above a faster twist, or lighter bullet, increases the distance between entrance and tumbling.

 

Intended usage also plays a role in bullet selection. If you are going to punch paper the bullet weight, and type, might be different than one used for self defense. Or if you are just blasting steel targets you do not need to worry about twist rates or bullet type.

 

BTW, the original incarnation of the M16/AR15 used a 20" long 12 twist barrel with a 55 grain bullet. This was the ideal combination for what the AR was originally designed for which was a short, <300 yard, range weapon. It was never designed to be a long range caliber and the military has ruined the caliber trying to turn what is a short range caliber into a long range caliber. The original design was devastating because the bullets would fragment very quickly. The Vietnamese people are very similar to a typical Afghan but because we now use a 7 twist barrel rather than 12 twist 55 or 62 grain bullets are passing through Afghans before tumbling. In Vietnam a 55 grain bullet, with a slower twist, would tumble and fragment before exiting resulting in a totally different wound channel.

 

If I had to pick a single twist for anything and everything I might encounter it would be a 9 twist. It will shoot almost everything well and the only thing it cannot do well is shoot 77+ grain bullets. But realistically a 55 or 62 grain bullet is the most likely weight you are to encounter and the 9 twist shoots those just fine. A 7 twist may, depending on bullet construction, might cause the bullets to fragment in air from centrifugal force.

[gregintenn]

I have ARs with 1:7, 1:8, and 1:9 twist barrels. They all shoot 55,60, and 62 grain projectiles well. I've not tried anything lighter nor heavier. I don't see a big difference.

 

Now that I think about it, I did try some 77 grain stuff in a 1 in 9 twist barrel. It hit the target sideways.

Edited July 7, 2015 by gregintenn

[ArmyBrat61]

Dolomite.....you really need to write a book. There is just too much info on too many subjects you have stored with in you...and I appreciate every single bit of it. I am much more comfortable with the AK....but the wife and times being what they are....demand a change. I am not too old to learn this info....just behind the curve...thanks gentlemen.