In a prior topic, it was posted that building an AR without checking head-space was inviting disaster. I have heard the other side of the coin many times that it wasn't necessary because components are built to specifications with tight tolerances. I share the later view and have shot rifles (not ARs) with significant headspace issues which only resulted in split brass. Regardless of your view point, I thought of a way to check my new-build AR's headspace with no tools. When my BCG came in today, I took the BCG apart. I placed a new piece of brass with a flush primer installed into the chamber, and inserted the bolt (don't use a case loaded with powder and a bullet to do this - only a primer). That told me I had enough headspace especially since I could do it by hand. Then I used the firing pin to measure the freeplay/headspace tolerance of the chamber by sliding the firing pin into the back of the bolt and letting it touch the brass. The plunger forces the brass forward into the shoulder leaving any freeplay at the bolt's face. The difference between the firing pin flush with the bolt face and with the brass in the chamber is your freeplay. You can actually see the difference in the gap between the flat on the ring stop of the firing pin and the back of the bolt. If there is no gap, you have too much headspace.