This has probably been asked before but I could not find it. Is it even remotely possible to fire much less chamber a 300 blkout round in an upper that has a 5.56 barrel? I have (had) a diamondback complete upper on a bushmaster complete lower. Loaded magazines with federal 5.56 lake city ammo out of the can and after 30 rounds the rifle had a catastrophic malfunction. Bolt carrier split, and is stuck in the bowed out and cracked upper. Blew the magazine out and apart. Sent the upper back to the manufacturer and they said it had a 300 blkout casing in it and they think the bullet is still in the barrel. I do not beleive the round will go foward into the barrel enough the allow the bolt to come forward enough to acctually fire a 30 cal round. Am I wrong on this? I do have a 300 blkout rifle but no 300 blkout ammo. They do not even sell the complete upper I bought from them in December (9"rail). I don't know what to do or beleive. The federal ammo was also sent back to federal on there dime which is great customer service. They said they would test the ammo and give me the resuts. I think it was a 5.56 round in which the round split. I will get the diamond back upper back in a few days. Let me know what you guys think.

beachdoor

I got nothing.

With certain bullets and deep seating depth or light crimp, yes, it is possible.

You end up with an extruded bullet as a squib and and at minimum a case failure, at worst, a blown up gun.

Who knows in this case, you may have had a split case that somewhat resembled .300.
Or Diamondback is trying to screw you.
Or there really was a .300 round in there.

Can_you_shoot_300_BLK_out_of_a_223____Update__It_s_Out___Page

http://lmgtfy.com/?q=300+blackout+223+kaboom

Yes, it will chamber with bullet setback.