SBRing a 300/221 - how short?
 

I'm about ready to send in my SBR paperwork on an AR (starting with a 22LR build), and am wondering about shortening my 300/221 upper. I have a 16.5" 1:8 twist upper now with the gas block in the pistol position.

Some questions....how short can I go with a 1:8, and still get stable subs? (don't want a baffle strike) I'm assuming I could probably put more powder behind a 220 and still stay subsonic?

Any harder to get subs to cycle the action with a shorter barrel?

I think most supressor manufacturers recommend 10" minimum to avoid any stability problems. I am going 10.5 to use with my AAC762SD. eta: Plus the shorter the barrel is, the quicker the blast baffle erosion is going to be.

8 to 10 inches shouldn�t be a problem, but your gas port size will determine some of this. You could always open the port up and use an adjustable block to fine tune the gun. I wouldn't worry about erosion with subs.

As to how short you could go with an AR-15 shoooing a 300 whiper/221/fireball/wtf I'd say the practical short limit is about 4". That's breach face to muzzle. I just ran a Quickload simulation with the following assumptions.
Case 300/221
P MAx = 50763 psi (3500 Bar).
cartridge OAL with a 220 SMK 2.28" (fits AR-15 magazine).
I let Quickload select suitable powders. From the list I picked IMR 4227. It gave a 100% case fill with a charge charge weight 12.34 grains to give 1050 fps MV.
calculated peak pressure: 46131 (about 91% p-max)
muzzle pressure: 15214 psi
Efficiency: 22.7%
The actual distance the bullet moves to exit the muzzle is only 3.2".

A longer barrel be more efficient and have lower muzzle pressure. A 10" barrel would only need 9.55 grains of the same powder for the same velocity with 29% efficiency with a 3960 psi muzzle pressure. (per Quickload).
I expect it could be made to cycle. Where to drill the gas tube hole and how long of gas tube to use would be the question. There should be enough energy available.

The powder isn't completely burned in the barrel though I'd expect a suppressor rated for 308 WIn would last a very long time.

I made 6.2 inches and it seemed ok.