OP, just measure the tab on the back of the upper receiver - much easier. That's described in the thread, and has a picture. <thumbsup> That gives you a height-over-bore measurement, but from the top of the BCG instead of from the center of the BCG. Also, when doing precision measurement, the first thing you do it calibrate, or check, your measuring equipment. Not saying this is the case here, with you, but if you're coming up with a non-standard measurement, the first thing you go back to is the measuring device, and check it, before going any further.