Thanks to everyone for clarifying this topic so well. [my geometry is pretty rusty - Michael, you're giving me a good refresher!
And, I apologize if I've placed this topic incorrectly - I understand why Rich wants to keep this forum well organized, and I'll do my best to cooperate. On the other hand, while MOA would usually be used regarding rifles, wouldn't it also apply to pistols, and perhaps even theoretically to shotguns firing slugs? It is a general concept of firearms accuracy, no? As olegunftr pointed out, it seems this concept would even apply to an M1A1.