I've pretty much adopted a gunsmithing attitude towards aftermarket replacement parts. In gunsmithing, you can't just plop new parts into a gun without risking serious injury. Hammer and sear faces must be matched, new barrel chambers precision reamed and checked, bolt lugs lapped, bolt faces checked, firing pins fitted and length checked, ...etc.
As a member on another machinists forum said:
"Sometimes you buy a tool, what you get is an unfinished kit"...