I did mine as a 5.56 but using a .300 blackout would be the same.
I built a pistol lower and put together a 10.5" Noveske upper. After test firing and making sure everying worked as it was supposed to, I finally got my stamp back for my supressor. Shot it as a pistol to make sure the everything functioned the way I wanted it to. Once I was happy with it, I permanently attached the supressor.
As a pistol:
I would agree with using a 9mm can as well for a .300 blackout. I've seen some cheap ones for sale and they are usually about 100-200 cheaper than a .30 can.
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