300 Blackout SBR questions

[esh21167]

Looking at doing a 300 AAC SBR.

First, can I use a .223/5.56 lower or must it be marked multi-cal? I think I found this answer that the lower markings are irrelevant so long as the barrel caliber marking matches the Form 1 caliber. Then I can switch uppers at will, just keep the original upper so the registered configuration still exists/can be set up.

Second, what is/are some decent barrel lengths to consider as far as reliability? Probably will suppress at a later date. Thanks.

[Nate7667]

Looking at doing a 300 AAC SBR.

First, can I use a .223/5.56 lower or must it be marked multi-cal? Yes, 556/223/multi markings are fine. Just mark your Form 1 with 300Blk NOT MULTI

I think I found this answer that the lower markings are irrelevant so long as the barrel caliber marking matches the Form 1 caliber. Then I can switch uppers at will, just keep the original upper so the registered configuration still exists/can be set up. Yes, you can swap uppers with no issue

Second, what is/are some decent barrel lengths to consider as far as reliability? Probably will suppress at a later date. Thanks.

I would look at 8.5-10.5" barrels. Anything longer, to me, is a waste (of $200 and 4 months) for doing a 300Blk as an SBR.

[LMTguy]

I went with a 10.5 on my 300 BLK pistol just because I wanted a longer rail.
Hopefully this helps
screenshot20120318at110.jpg

[ExFlyinguy]

Unfortunately I can't share a link at the present, but from what I've read- On your form 1, ATF won't accept it if you list "Multi Cal" but you can list more than one caliber, i.e. put 5.56 & 300 BLK.

ETA: As far as barrel lengths, I have a 10.5 and it works fine with a can. - It will not cycle subsonic 300BLK without a silencer.

[LMTguy]

ETA: As far as barrel lengths, I have a 10.5 and it works fine with a can. - It will not cycle subsonic 300BLK without a silencer.

Have you looked into adjustable gas blocks?

[ExFlyinguy]

Have you looked into adjustable gas blocks?

No I haven't. Since I use mine with a can it is a non issue, but that may well help someone else. Thanks.

[NathanB]

I went with 8.5 for a few reasons. Mine is meant to be shot suppressed and that's it's intended use so velocity wasn't so important to me and I wanted it short. That way I can throw a 13 inch rail on it and the can wil fit inside and still be manageable

[esh21167]

Thanks for input! I have a 5.56 PSA upper with the FN barrel, but their 300's with FN barrels are almost $600. Just starting to look at 300. Some quick searching shows Andersen has a 10.5" 300 AAC upper with an adjustable gas block for about $360. That seems reasonable. So an adjustable GB would most likely be able to be tuned to function/cycle with or without a suppressor?

[Nate7667]

Normally the only reason a 300blk won't cycle without a can is when you use sub sonic ammo. If you use subsonic ammo with out a can, you deserve to have to manually cycle your action.

In my opinion an adjustable gas block is a waste of money on a 300blk upper.

[esh21167]

Normally the only reason a 300blk won't cycle without a can is when you use sub sonic ammo. If you use subsonic ammo with out a can, you deserve to have to manually cycle your action.

In my opinion an adjustable gas block is a waste of money on a 300blk upper.

Got it. Makes sense to only use subsonic suppressed. Thanks again.