The parent cartridge of the BLK is 5.56 NATO. You can use everything off a standard 5.56 AR-15 for a BLK rifle except the barrel- even 5.56 brass can be cut down and resized for use in your Blackout rifle.
kR-15.com reader, reloader (and now co-author as he is quoted below) RBC recently shared the challenges he ran into trying to make his own BLK brass. It's helpful information for anyone who makes their own brass from 5.56 cases.
RBC was having very frequent failure-to-chamber and jamming problems with his cartridges.
“I’ll get one or two round that feeds and chambers perfectly, and then get 4-5 that feed, but won’t chamber, and seize up hard in the chamber. A real booger to get out without breaking anything. all rounds seem to feed well, but fail to fully chamber and lock the bolt.
Cases were trimmed to 1.358, the middle of the tolerance, and that specified for the Barnes 110 GR Tac-TX, and verified by at least one commercial re-formed case producer I’ve talked to.
I’ve used PCT go and no-go gauges to check headspace, as well as the Wilson case gauge, all of which checks out, and says everything SHOULD fit.”
When specs are verified by multiple sources and you have eliminated some possible causes it's time to be methodical: double-check your specs and flex those troubleshooting muscles. That's exactly what RBC did:
“The SAAMI cartridge length specification calls for 1.368 +0.000 -0.020, so the lower limit would be 1.348. 1.358 is right in the middle. As I said, Barnes calls for 1.358 shell case length for their highly regarded 110GR Tac-TX. So I milled the brass down to 1.358 +- 0.001, deburred, and cleaned it.
When I was now getting a 50% failure rate chambering when using my Lee dies, I went and bought a set of RCBS small base dies, thinking the cartridges just needed to be sized smaller than the Lee dies, and then fire formed to fit my chamber.”
This still didn't solve the problem. The dies are now officially eliminated from possible causes. The Go/No-Go gauges covered the chamber….how about the brass itself?
“The brass was cut down from some once-fired PMC .223 shells I had laying around forever.“
The type of brass you use can make a HUGE difference because of the case wall thickness. When you cut down 5.56 brass to 300 BLK length case wall thickness variance becomes case neck thickness variance. Experience varies with the brass but PMC and S&B are known to have this problem in certain lots. I have PMC resized brass I purchased commerically – it works great but I don't know if it was presorted or not.
“If you have a .308 bullet, seated into a cartridge with 0.015 thick brass at the neck, then you end up with .338 outside diameter at the case mouth, when it should be .334 max. Houston, we have a problem. So I separated out all the brass with a thickness of 0.013 or less, loaded them up, and, lo and behold, every one of them feeds and chambers perfectly. Problem solved. [Insert big sigh of relief here]”