OK I'm new so be gentle I'm totally new at building a rifle so here goes, What is the difference in lower recievers? What I mean by that is I've been reading and 1 person is using a "ABC" receiver and someone else is using a "XYZ" receiver? What is the big differences besides the name? Are some better quality then others, is it only because thats what was available at the time ect. (this is where I'm going to get HUH?) Arent they all relativley the same? Or does ABC mate better with a DEF upper then XYZ does? I would think they would all have to meet certain spec's to be sold. I don't want to drop a fortune on a name, but I do want to understand what it is I should be looking for in general.