THREADED muzzle, fluted, etc. All you have to do is screw them into your upper receiver, get whatever configuration AR you WANT, $100-$200. takes about 5 minutes, no special tools at all, basically, altho a "field' headspace gauge is nice, it's rarely needed, especially if you handload. Put about half a charge of the slowest burning 223 powder in a case, fire it, look for problems shown by the condition of the fired case, work up, miking the case, etc.