twist rates and short barrels

[QuarterCorner]

Greetings Forum,

I'm looking to build a short barrel AR for fun and shorter range hunting, I have an Armalite AR10 that I want to slap a new short barrel on and swap the furniture. Looking to hit elk and steel targets out to 300 yards at the furthest. I have a 7mm mag bolt gun so a short woods rifle is a fun concept.

Armalite has their 13.5" competition upper in 1:10" twist rate, while a 13.5" barrel has a 1:8" twist rate. Now I've been told by reviews on the Federal website that shorter barrels do better with lighter bullets, which also need a gentler twist to get stabilized. I've also read that over-twisting bullets on a .308 AR is not too detrimental for accuracy. All the charts I've seen show that a 1:8 twist rate is needed for bullets heavier than 200gr, while the short barrel AR's excel with bullets in the 150gr neighborhood.

So my questions are as follows, what's the reasoning for cutting such a fast twist into the barrel and should I even be concerned about it?

[jtallen83]

For 300 yards and in I wouldn't be concerned. I've done some research on the 1:8 twist stuff, they do better for the heavies when you run the math but it isn't an earth shattering difference plus finding and paying for the odd twist rates is another issue. I have a Sig P716 with a 12.5 inch barrel and am in the process of using one of there 12.5 inch barrels on a DI build. I have not seen a big difference in bullet weights, at least nothing more than I have seen with other lengths, every rifle will have its particular favorite. I do really like the 130 gr. MK 319 load but then it was designed from scratch for short barrels and barrier blind penetration.

[shepp]

2 hours ago, QuarterCorner said:

Greetings Forum,

I'm looking to build a short barrel AR for fun and shorter range hunting, I have an Armalite AR10 that I want to slap a new short barrel on and swap the furniture. Looking to hit elk and steel targets out to 300 yards at the furthest. I have a 7mm mag bolt gun so a short woods rifle is a fun concept.

Armalite has their 13.5" competition upper in 1:10" twist rate, while a 13.5" barrel has a 1:8" twist rate. Now I've been told by reviews on the Federal website that shorter barrels do better with lighter bullets, which also need a gentler twist to get stabilized. I've also read that over-twisting bullets on a .308 AR is not too detrimental for accuracy. All the charts I've seen show that a 1:8 twist rate is needed for bullets heavier than 200gr, while the short barrel AR's excel with bullets in the 150gr neighborhood.

So my questions are as follows, what's the reasoning for cutting such a fast twist into the barrel and should I even be concerned about it?

Welcome to the forum feel free to tell us about your self in the intro section

[98Z5V]

4 hours ago, QuarterCorner said:

Armalite has their 13.5" competition upper in 1:10" twist rate, while a 13.5" barrel has a 1:8" twist rate.

If you look at the Armalite website, it indeed does states that specific barrel is 1:8" twist. 

https://www.armalite.com/product/ar10-308-13-5-target-barrel/

The website is wrong.  I've got that barrel, and it's a 1:10" twist.  I've contacted them about the website error, and they don't seem to care. 

Word of caution on that barrel:  Do you plan to Tax-Stamp SBR this rifle, through the AFT?  If the answer is no, then you will have to use Armalite's Competition Brake, pinned/welded, in order to bring this barrel to the 16" minimum length.  Other brakes, comps, muzzle devices won't get it to 16". 

[jtallen83]

48 minutes ago, 98Z5V said:

Word of caution on that barrel:  Do you plan to Tax-Stamp SBR this rifle, through the AFT?  If the answer is no, then you will have to use Armalite's Competition Brake, pinned/welded, in order to bring this barrel to the 16" minimum length.  Other brakes, comps, muzzle devices won't get it to 16". 

There's always the pistol option. The new SBA3 is a pretty inviting option.

https://www.sb-tactical.com/product/sba3/

[98Z5V]

5 hours ago, QuarterCorner said:

 I have an Armalite AR10 that I want to slap a new short barrel on and swap the furniture.

 

1 minute ago, jtallen83 said:

There's always the pistol option. The new SBA3 is a pretty inviting option.

https://www.sb-tactical.com/product/sba3/

He can't turn it into a pistol now, brother - it's already a rifle.  A new lower would do the trick, though.  Once a pistol, always a pistol.

[jtallen83]

Just now, 98Z5V said:

 

He can't turn it into a pistol now, brother - it's already a rifle.  A new lower would do the trick, though.  Once a pistol, always a pistol.

Missed that, you are correct as usual.

[98Z5V]

I think that brace is gonna go on my Shockwave, though.  Fired a Shockwave with a brace this morning...  MAN!  What a difference!...

[Robocop1051]

1 hour ago, 98Z5V said:

Word of caution on that barrel:  Do you plan to Tax-Stamp SBR this rifle, through the AFT?  If the answer is no, then you will have to use Armalite's Competition Brake, pinned/welded, in order to bring this barrel to the 16" minimum length.  Other brakes, comps, muzzle devices won't get it to 16". 

Shouldn't the PWS CQB or the Noveske KX3/5 make a 16" OAL barrel?? 

[98Z5V]

I think those are both 2 3/4" OAL.  Somewhere in the build thread on my gun, I listed that brake, long as hell. Just barely for the 13.5" to a hair over 16".

30-cal PWS CQB Comp is 2.48" OAL.  30-cal KX3 is 3.28" OAL; KX5 is 3.25" OAL. Those two will do it.

That's the reason you see so many .308 barrels that are 13.7".  Something 2.5" long will get it legal.  Gotta be careful with this 13.5" barrel.

Edited July 22, 2018 by 98Z5V

[98Z5V]

In this thread (linked below) are all the details on the brake, overlap of barrel threads (really needs to be considered on these 13.5" barrels), and all the other details on the build.  OP, this has to be close to what you're intended on doing. Can't be far off, that's for sure.