Can any lower be registered as an SBR? After the stamp and engraving of course. I'm only wondering because I was thinking of doing one on a Plum Crazy Lower in a while. Just didn't know if the lower has to be built from the ground up. Hopefully that makes sense.
To get to the crux of your question, OP, back when I was selling lots of PC lowers, I did the drilling on a PC lower, registered it as an MG and then Form 5'd it to PC in Havasu for FA testing. They ran thousands of rounds (unsure of the exact number) but when I got it back the whole lower was full of powder residue so it was apparent that they ran, ran, ran and ran it, with and without a suppressor, judging by all the residue in it.
There was no degradation or enlargement of the sear hole. There were no cracks. There was no breakage whatsoever.
They ended up giving it back to me and I ended up selling it for 250 bucks to another C2.
So, before the King of the Trolls "sinks your battleship" with his snide comments, based on nothing, no empirical experience whatsoever, ask the folks who've "done it". If you want to make an SBR on a PC lower, there's not a reason in the world why you can't, and nothing but folks opinion
, based on diddly, why you shouldn't....