Headspace isn't something you'll assemble, or worry about getting right, on your end. The barrel manufacturer has to get that right, when they build the barrel. You have a couple options - buy headspace gauges and check the headspace - if it's not right, you'll need a good AR gunsmith to get it right for you. Usually involves reaming the chamber. Buy a bolt that's already been headspaced to your barrel. Most of the time, this is a service that those selling barrels will provide for you. Research which ones will do this, and make your barrel purchase from them. Hint: Rainier Arms and Fulton Armory are two that will do this.