simple explanation....they make a small precise gauge that measures the space between the bolt face and the chamber. too much or too little space and you can have big problems. imagine the round not being fully seated in the chamber and still firing off. kaboom! not a big deal with a milspec AR15, but 308s have no standard spec sheet to machine off of so using different manufacturers parts is a crap shoot. in your case, you already have parts that were not meant to work together you have modded. nothing wrong with that....and I applaud your creativity getting it running, but I wouldn't fire a single round again until I had it headspaced by a smith, or orderd the gauges and do it yourself. you are gambling with your face/eyes/fingers here man.