Interesting post. Yes, from the metals industry lead does harden all the way through and not a surface hardening like steel alloys. Does the BHN tester measure only surface hardness? I can't answer that other than to say the metals industry uses similar testing though of course with far more precise (and expensive) test equipment. Do bullets work soften when fired? Another good question, I would assume so but given the millisecond the bullet is in the bore would work softening effect anything? Dunno, don't know how one could test that but it is an interesting mind teaser to think about it.
Rick