I'm looking at building/getting an AR-10 at some point just because. I understand that there are several makers just like the AR-15 but not all are compatible with each other. Can someone explain to me what the most popular/available variants are so I don't end up getting the Studebaker/Edsel of the AR-10 family instead of a Ford/Chevy.

An armed member in a country is a citizen, an unarmed member is a subject.