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For the price of a USED A3 upper you could put $100-125 with it and get a brand new unit. It's worth the expense if Ar's are new to you since you will not want to deal with diagnosis issues for problems. Spend time learning how to shoot and break the gun down along with reading on this site to get more familiar.

I only buy used parts when its a steal since most of the time its a crap shoot on condition.