I'm new to the semi auto build and this year I want to build a few ar's, if you guys would stop buying all the parts . For the first one I have a complete CMMG lower, and a DPMS stainless upper without the internals. Now, some people tell me you can just slap the upper to the lower and you are good to go while others tell me I need to take it to a gun smith and have it head spaced (?). I have a carrier assembly that came out of an Armilite ar that I was going to use. I'll do some searching on this site, but until then I was hoping you guys could educate me on the complete build concept and explain some of the do's and don'ts involved in these builds. Thanks.