I'm waiting on some stamps, as soon as they come in I'll be replacing an HBAR upper with a new short .300BLK upper. I intend to reuse the same bolt and carrier in the new upper. The bolt has maybe 2000 rounds on it, tops. Likely less. Bolt was sold as being MPI and all that good stuff.

Searching seems to indicate that I am probably OK to reuse the bolt. Checking headspace would be a good idea but not strictly necessary.

Am I doing anything needlessly foolish or dangerous?

Reusing the bolt is fine since both cartridges share the same bolt. I would disassemble the bolt and check headspace when the new barrel comes in, though. Probably not totally necessary, but it's an easy thing to do, and certainly can't hurt for the sake of being safe. Post pics once your stamp comes in.

"- am I going to die?" Yes, and me and everyone else, no one gets out of this game alive.  However there would be no problem reusing the 223/5.56 bolt as it's the same for a AAC 300 Blackout.

Thats what I was thinking. Any AR barrel and AR bolt *should* work together. However I'm dealing with mid-market vendors so maybe something was right at the edge of tolerances, the old bolt and barrel wore in together, and now we're out of spec on a new barrel and new caliber.

I'll scrounge up a field gauge. The other possibility is that the ATF doesn't get my stamp back and I can get a deal on a bolt or BCG at NRAAM.

Why not just get a new bolt for $55 and call it good?

ETA: back-up bolt/FP/retainer/cam kit here.

Picked up a DSA BCG at NRAAM for $85, problem solved.