Another angle to this on the AR side, that I've seen in classes. For a while Colt was making AR's with larger hammer and trigger pins, to foil Bubba's Big Plan to use M16 parts. Far as I know this was only Colt, but I've seen a few guns that somehow over the years, got put back together with small pins. This is several malfunctions waiting to happen, including fire on release, so-- something else to look for.

It should go without saying, but some people don't appreciate the gravity of it-- don't have any select-fire parts in your trigger group. There are probably a great many guns out there, especially home-builts from twenty-plus years ago, that have some parts in there that ought not to be in there, unbeknownst to the owner. Know the dif and check. I'll try to get some pics up in the next few weeks as I will be looking in on some department guns.