I am planning on building my first AR over the winter probably. Right now a friend of mine, who is an armorer, has built an AR15 carbine for me and almost done withe an AR10 in 308 for me. But why should he have all the fun. Im trying to understand all the little details about doing this but it brought a thought to mind. Say I decide to build and AR. So when I am buying all the parts for it how am I going to know what will give the right head space? I mean I get what it means but why would some parts work and others wont? Further what is typically the root cause of headspace being to far off?