20" ar-15 build ?

[z0mbies fear me]

I've decided that I "need" another ar-15 but this time I'm thinking about going with a 20" barrel. While barrel shopping I have found some 20" 5.56mm barrels with twist rates of: 1 in 7", 1 in 8", 1 in 9", and 1 in 12".

My main question is: What twist rate is typically best for which weight bullets?

Primary purpose of said rifle will be range toy with a backup role of varmint slayer. This will be assembled on my CMT/TGO upper.

[cz75b]

faster the twist rate the heavier the bullet it stabilizes.so 1:7 good for 80ish gr,1:12 for the really light stuff 30-40 gr varmint stuff

[z0mbies fear me]

Thanks, that was what I was thinking but I didn't know for sure.

I'd like to get something that will do up to 62 grainers as that is what I generally shoot but I'd like to be able to some of the lighter rounds for groundhog control.

[TrickyNicky]

I've got a 1/8 twist on my 20" and so far I've been pleased with it. 

[Ted S.]

I'd like a 1:8 for my build.

[Clod Stomper]

If you're not going heavier than 62 grain, I wouldn't go with a faster twist than 1:9.  That said, I've shot lightweight (50 gr) varmint bullets through a 1:9 20" barrel and they spun apart in the air.  That won't happen with FMJs or most hollowpoints, but it's a possibility.  

 

Were I going to use it as a varmint rifle and rarely go over 55 grains, I'd go with the 1:12.

[Guest tangojuliet]

1:8 is the way to go imo

[btq96r]

Another consideration is the 5.56 caliber is most effective when it tumbles and fragments. If your twist is too fast the bullet will not tumble and will not fragment substantially reducing its effectiveness. And the bullet must be going ~2,650 fps for it to fragment. So if you have a barrel that is a 7 twist and you are shooting bullets that are too light (like 55 or 62 grain), and too slow when they hit the target it will act just like a 22lr because the bullet will not tumble and if it doesn't tumble it will not fragment. This is why we are hearing about all the problems overseas. The 7 twist M4, with its short barrel, is the worst of both worlds. The twist is too fast for the 62 grain bullet to reliably tumble but even if it did the 14.5" barrel means the bullet will not fragment if the target is beyond ~75 yards. In Afghanistan most engagements are well beyond 75 yards, or even twice that, so when they do hit their target the bullet is acting just like a 22lr and punch a clean .22 hole which is easily survivable unless the hit is in critical areas.. If they wanted the M4 to be effective they need go back to a 9 or 12 twist. If they wanted to be even more effective they need to do like the Marines and go back to a 20" barrel to extend the range at which the bullets will fragment.

 

BTW, the reason why the military adopted the 7 twist was so they could shoot the 62 grain tracer bullets which are about 3/16" longer than a all lead 62 grain bullet. It was not to improve long range accuracy or effectiveness. A 9 twist will shoot the heavy bullets just fine and are more effective than when shot out of a 7 twist because they will tumble and are generally more accurate.

 

The entire post is good info, but the quoted points are very important to consider in the whole which twist for my AR craze.  Too many people get it in their head that Mil-Spec is what they need, because if it's what our troops are carrying, it must be the right answer.  Not always.

 

Keep in mind, that the military solution is usually based on the need to make weapons parts uniform across hundreds of thousands of systems and reduce the maintenance requirements in the field.  Barrel swaps for specific missions isn't something an infantry company can do while living out of a rucksack.

 

As Dolomite breaks down, the 1:7 twist was introduced because of the tracer rounds.  Quite frankly, with the current state of night vision and IR target pointers/illumination devices, I really feel the tracers aren't worth the disadvantages listed above that a 1:7 twist brings.  The over-penetration issue is more of a problem in Afghan than it was in Iraq, but I think there are plenty of Afghans that any round/twist combo will run through.  Moving around the mountains while living only on a handful of village baked bread and rice will keep you slim.

Edited February 11, 2015 by btq96r

[Jct1911]

I run 1/8 in all my newer guns, but never drop below 55grn bullets. My best results as are many others are with bullets weighing 70 +grs and pushed hard.

[z0mbies fear me]

Well after reading all of these replies and doing a little more research I went ahead and ordered a 20" 1 in 12" twist Green Mountain Rifle Barrel. Also went ahead and ordered everything else I needed for the build

[Spots]

I will add this. T years as a Marine and we started on A2's in boot camp then got issued A4's in the fleet. While the 16" collapsible stocked carbines are all the rage in the civy world and with all other branches, the Marine Corps is big on tradtion. I loved my A4. Yeah it makes vehicle and helicopter stuff more of a pain in the boot, but when it comes time to accurately put lead down range out to 500 yards, I still feel most comfortable with a full stocked 20" AR instead of a 16" carbine. So enjoy yourself.

[Clod Stomper]

I will add this. T years as a Marine and we started on A2's in boot camp then got issued A4's in the fleet. While the 16" collapsible stocked carbines are all the rage in the civy world and with all other branches, the Marine Corps is big on tradtion. I loved my A4. Yeah it makes vehicle and helicopter stuff more of a pain in the boot, but when it comes time to accurately put lead down range out to 500 yards, I still feel most comfortable with a full stocked 20" AR instead of a 16" carbine. So enjoy yourself.

 

I like a compromise.  I'm leaning toward 18" barrels now.  16 is just too short in my opinion.  Barks too loudly, loses too much velocity.  But I have both 16 and 20 inch rifles.  So my next one will be 18".  

[McGarrett]

I used this on my 20" TGO AR build; http://www.gmriflebarrel.com/gm-m16-m16a2-5-56mm-20-barrel-with-extension-1-8-twist/

 

[URL=http://s36.photobucket.com/user/McGarrett50/media/photo2_zps26670907.jpg.html][/URL]

[z0mbies fear me]

Everything but the barrel should be here tomorrow and the barrel should arrive on Thursday. I'm getting excited now.

[jcluff]

Dolomite, I have been trying to find data that confirms you assertion that twist affects tumble and fragmentation within a body or flesh. The only thing I can find that provides any data one way or another is the post buy molon on arfcom. Do you have any links I can read?

http://www.ar15.com/archive/topic.html?b=3&f=118&t=512635


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[btq96r]

I will add this. T years as a Marine and we started on A2's in boot camp then got issued A4's in the fleet. While the 16" collapsible stocked carbines are all the rage in the civy world and with all other branches, the Marine Corps is big on tradtion. I loved my A4. Yeah it makes vehicle and helicopter stuff more of a pain in the boot, but when it comes time to accurately put lead down range out to 500 yards, I still feel most comfortable with a full stocked 20" AR instead of a 16" carbine. So enjoy yourself.

 

16" is all the rage on the civy side because that's as short as you can get without an NFA tax stamp.  The Army & Air Force M4 carbines actually have 14.5" barrels.  Not enough difference for most folks to go through the NFA process to be exactly like the kit our troops are carrying.

 

I agree with you that the Marines keeping a 20" rifle is a good call, but the Army really only trains out to 300 meters, so the M4 is good enough for that.  Anything past 500m is covered by crew served weapons and by what they call squad designated marksman, who carry a tricked out M14 with a new stock and some good optics on it.  Every infantryman I talked to who did that job loved their M14, and could put rounds on target at a good distance.  They were very useful in Afghanistan during long range engagements in all that open terrain.

[Spots]

16" is all the rage on the civy side because that's as short as you can get without an NFA tax stamp. The Army & Air Force M4 carbines actually have 14.5" barrels. Not enough difference for most folks to go through the NFA process to be exactly like the kit our troops are carrying.

I agree with you that the Marines keeping a 20" rifle is a good call, but the Army really only trains out to 300 meters, so the M4 is good enough for that. Anything past 500m is covered by crew served weapons and by what they call squad designated marksman, who carry a tricked out M14 with a new stock and some good optics on it. Every infantryman I talked to who did that job loved their M14, and could put rounds on target at a good distance. They were very useful in Afghanistan during long range engagements in all that open terrain.

Yeah I knew the M4's were 14.5", our Staff NCO's and officers carried them. And I love me some Sage stocked EBR goodness. I built a Troy stocked M14 last year and still miss and debate building an M39 replica a lot. While I agree the M4 is more than adequate to 300 yds, I just shoot better with a 20" gun.

[Dolomite_supafly]

Dolomite, I have been trying to find data that confirms you assertion that twist affects tumble and fragmentation within a body or flesh. The only thing I can find that provides any data one way or another is the post buy molon on arfcom. Do you have any links I can read?

http://www.ar15.com/archive/topic.html?b=3&f=118&t=512635


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http://stevespages.com/pdf/5_56mm_military_info.pdf

[Oh Shoot]

16" is all the rage on the civy side because that's as short as you can get without an NFA tax stamp.  The Army & Air Force M4 carbines actually have 14.5" barrels.  Not enough difference for most folks to go through the NFA process to be exactly like the kit our troops are carrying....

 

Especially since you can do a 14.5" barrel with welded muzzle device to get to almost exactly same overall length as military without the stamp. Will still be just a skosh longer since standard military birdcage won't quite get there, though.

 

- OS

Edited February 18, 2015 by Oh Shoot

[Caster]

9 twist will do just about anything.  

I have a CZ527 in 223 with a 9 twist and it will run 75g HPBT into a quarter at a hundred all day long.  it's actually pretty boring, it does it so well.

[jcluff]

http://stevespages.com/pdf/5_56mm_military_info.pdf

interesting article. Makes a very good case for using 77gr for defensive purposes. However, I couldn't find any info in it about twist rate and its effects on bullets tumbling to flesh or similar mediums.


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[z0mbies fear me]

Some of the parts came in today... pics to follow.

[z0mbies fear me]

[z0mbies fear me]

UTG PRO Model 4 15" Rifle Length Super Slim Free Float Handguard

Timney 668-S Competition Solid AR-15 Trigger 4lb

Daniel Defense .625 Clamp Low Profile Gas Block

Seekins Precision Billet Magazine Release Button

Anderson Manufacturing A2 / Rifle Length Reciever Extension

Anderson Manufacturing Rifle Length Buffer - 5.2oz

JP Tuned and Polished Buffer RIFLE (A2 style) Spring JPS-OSR

Magpul MOE Fixed Rifle Stock

Magpul MOE pistol grip

Yankee Hill Machine Rifle Length Black Gas Tube

JP Enterprises Thread Protector TP1/2X28 .750B

Ares Armor nickel boron bcg

M16 A1 5.56mm 20" 1:12 twist 20" Barrel from Green Mountain Rifle Barrels

BCM gunfighter charging handle (small latch) Edited February 23, 2015 by z0mbies fear me

[z0mbies fear me]

Just waiting on the barrel and a charging handle