Gas systems

[DNP]

This is where the block sits on a 16” with rifle gas. 14.5 is not gonna fly. 

[98Z5V]

11 hours ago, MR308 said:

1.  Well, I hate to be the guy that digs up old posts like this, but I’ve got a new kind of conundrum. So I’m building a 14.5” .308 AR10. I’m having mcGowens make a barrel for it but I’m trying to find the best set up. The barrel will be pinned and welded with a Gamma VG6 and I have been planning on using a mid length gas system.

2.  I’ve heard back and forth opinions about twist rate and I was initially set on a 1:9 for added flight stability but I have people tell me that may be “too fast”.

3.  The other thing I’ve been trying to figure out is if I can cram a rifle length gas system on a 14.5 inch barrel. Based on what I’m reading here that’s a no due to not enough dwell time, but I wanted to figure out if I could. I don’t know the numbers to crunch but I was looking for someone who did.

My main thought process for this was that in a 14.5 the bullet would leave the barrel before all the powder in the casing has burned (at least that’s what I’ve read). So I assume that having a longer gas system would allow more pressure for a longer duration behind the bullet, this increasing muzzle velocity. Of course I don’t know a lot about ballistics as this is the first AR I’ve built and I’ve never reloaded, so this is all theoretical for me. Thanks for the help in advance!

1. I'd switch that "midlength gas system" right away, since they're building your barrel. Midlength Gas System is AR-15, and not .308AR.  There's a distinct Height-Over-Bore difference between the two, and using AR-15 gas tubes on .308ARs, with the higher Height-Over-Bore, means that you're "bending" the AR-15 gas tube up, to make it enter that .308AR upper receiver.  Not significant - but significant, in the details.  Have them make your gas port location for the Armalite AR-10 CARBINE Gas Tube...  And run the Armalite AR-10 Carbine Gas Tube in the gun.  It's got the proper Height-Over-Bore for a large-frame upper receiver, and it's actually a touch LONGER than the AR-15 Midlength Gas tube.   AR-15 Midlength gas tubes measure out at 11 3/4".  Armalite AR-10 Carbine gas tubes are 12 1/16" long...

2. This is all gonna come down to how heavy the projectiles are that you're gonna shoot, and what you realistically want this gun to do.  You're building a 14.5" gun.  How FAR do you want it to shoot?  I'd bang steel at 850 yards with a 14.5" .308AR, but I wouldn't HUNT that distance with it.  I've got 12.5" and 13.5" .308ARs, and they will go to the 850 target and hit it, repeatedly.  I wouldn't hunt either one of them past 300 yards, though.   They both only eat (my old handloads) of Hornady 178gr BTHP projectiles, and my new load of the Hornady 178gr ELD-X projectile.  Both are 1:10" twist barrels.  Your logic is right on the money, I commend you - shorter barrels usually need more twist rate, to keep up with longer barrels AND heavier projectiles.  The shorter you go for the Heavy, make the twist rate faster...  And that works, out to max distances.  If you're not handloading heavies, and just buying off the shelf ammo, then what's the heaviest ammo that you use in the gun?  That's really gonna determine how far you can accurately shoot the gun...  And that comes down to your realistic expectations of that you want this gun to really do...    If you're going for distance with it, on steel or paper, then you need to be shooting the heavies.  Faster wist rate will help.  If you want to hunt it, to max hunting distances,with heavies, then twist rate will help.  But, If I can hit a steel target that's 18" wide by 26" tall, at 850 yards, with a 12.5" barrel and a 13.5" barrel  - running 178gr handloads - then a 14.5" barrel will do the same, with the heavies.  State the objective for the gun, first, and we can line out the type of ammo that you're going to need to use to shoot that...  Hope that makes sense...

'3.  Never gonna happen.  I think @DNP's pic above, of a 16" barrel with Rifle Gas is the best example of that, on this entire planet.  There just isn't enough barrel on a 14.5" setup to even think about Rifle Gas.  Armalite AR-10 Carbine Gas System is the best way to go here.  I can get pics up of the 12.5" and 13.5", if you need them.  The 12.5" is running a custom 11.000" gas tube, and the 13.5" is running AR-10 Carbine Gas.

[98Z5V]

@MR308 - Read this thread, when you have the time:

 

[392heminut]

On 1/6/2021 at 1:09 PM, MR308 said:

What do you mean by “not long enough”? As in go rifle length gas system? And it is commonly 1:10 in most barrels, yes. Originally it was 1:11.25, and I was asking 1:9 for bullet stability reasons out of the shorter barrel

how are your groupings at 14.5” with a 1:10? - And how are they at range?

It's amazing what happens to a statement when a comma is left out!

On 1/6/2021 at 1:02 PM, unforgiven said:

.308 is 1:10. Mid gas barrel is not long enough. Just my 2 mag's

What he should have typed was "Mid gas, barrel is not long enough", and he meant 'go mid gas, your barrel isn't long enough.'

[unforgiven]

🤔😵

[98Z5V]

12 hours ago, 392heminut said:

It's amazing what happens to a statement when a comma is left out!

What he should have typed was "Mid gas, barrel is not long enough", and he meant 'go mid gas, your barrel isn't long enough.'

You just turned into Rene's Official Interpreter.  Don't worry, it's not a bad title to hold.  I'm @sketch's Official Interpreter - but I lose control alot of times...  

[392heminut]

7 hours ago, 98Z5V said:

You just turned into Rene's Official Interpreter.  Don't worry, it's not a bad title to hold.  I'm @sketch's Official Interpreter - but I lose control alot of times...  

You have a lot tougher job! Rene is pretty damn good at saying what he means, he just had a little glitch there!