My question to all is: I am doin my first AR15 .223/5.56 build and am curious as to which gauge I need?? I have read and compiled every answer you could imagine!! I am pretty sure all I need is the field gauge, and if the bolt closes on that, then it's way too loose and dangerous!! Although, I have also read that because AR components are built to spec, it is the one gun you don't have to worry about using a head space gauge at all!! I would greatly appreciate any and all input you seasoned builders have to offer, thanx!!!