One aspect of his weapon choice that is important is the weight associated with the ammo and the gun. He explains in detail that particular point. Being an old fart myself, running with extra weight is not desirable. Well running is not actually desirable. Most 7.62X39 uses 123 grain bullets plus a heavier case, so each round is twice the weight of one 5.56 x45 round using either 55gr fmj or 62gr fmjsc.