I found that later. From what I read, the feds took one of his .50 kits and intentionally damaged the bolt to make it slam-fire. They then used that as evidence that he was making "machine gun kits" in order to get the search warrant.
Not quite. The whole set-up was a little more complicated.
They didn't like him making the as-advertised under-the-radar .50BMG (NOT MGs) kits, so raided him on the "felon in possession" theory. ("Felon" arrived thru entrapment he wasn't smart enough to fight.) Having siezed some kits, they managed (through extremely tortuous logic & machineshop hackjobs) to get something to "fire" (through an extremely tortuous definition thereof). That got enough to justify the raid.
During the raid, they found some other stuff that was far more interesting: homebrew MGs. Not just cheesy hackjobs, but interesting new designs professionally executed.
Having 5 homebrew MGs to slap him up with, they dropped the slamfire .50s issue and ran with the 922(o) violation, with a side of "felon in possession".