From what I have been reading it seems like FMullegun, knows his shit. A lot of people seemed confused here with their information. Saying Bushmaster makes shitty firearms is ridiculous. They make great quality AR rifles! You cannot buy an M4 on the civilian market, unless you do the class 3 FFL paperwork and I'm still not sure that there are many actual M4s on the class3 market. You can buy a M4 style AR-15 though. The AR weapons system is the most practical and tactically modifiable weapons system on the market today. There are so many different names, styles, and variants, but they are all built around the exact same upper and lower receivers. If you own a actual M4 then it is select-fire and capable of fully automatic fire. If it's semi-auto only, then it's just an AR-15. If it fires .223, then it will fire 5.56 with no problems at all. There is almost no difference in the two rounds and their trajectory. Adding different length barrels and stocks does not make it a different firearm. There seems to be a lot of confusion about this. I can throw a body kit on my car and make it look like another vehicle, but it is still the same basic car. It's the same here. I built my M4 style AR up from Colt, Bushmaster, and Olympic Arms parts. The only difference from the military M4 is the select-fire capability and from the M-16A2 and A3 is the barrel length, stock size, and select-fire.