Also to consider between the M1A and the AR-15 is reliability. IMHO, the M1A is more reliable given its gas piston reloading mechanism which spares the chamber lots of heat and grime from the combustion of the powder whereas the AR-15 uses gas blow back which throws the heat and grime into the chamber, which has the potential for thermal expansion of the chamber around the bolt carrier and to put grit and grime in its way. If you are going to go 5.56, I would recommend the Sig 556 which uses a gas piston reloading mechanism and can accept the AR-15 magazine. It is though, more expensive at around 1800 and suffers a bit in long distance accuracy.