So I purchased a nice Colt preban AR-15 A2 lower recently, but I am having trouble finding a suitable upper. Part of my problem is that a lot people sell uppers in different stages of completion. I've seen stripped ones, some with a forward assist, some without, some with a barrel, some without, some have a BCG (I'm not even sure exactly what that is) some have a full auto BCG. I wish there was an "ARs for Dummies book" that I could buy. If I can't find a complete matching upper, I will do some research and buy the individual pieces. If anyone can explain the parts of an upper, that would help me a lot.