There is a headspace that is considered to be the "sweet spot" where you can shoot either one. Gus Fisher has posted this # a few times over on CSP and the M14 forum .I think it is 1.632 if memory serves me correctly .
BTW I shoot mostly surplus.Some is better than others,and some shoots in some rifles better than others,try to find what shoots best in yours for the $. SA,German,Portugese,Hirtenberger,Australian,Indone sian,Greek,Malaysian,RadwayGreen 7.62. Pretty much anything I can get my hands on cheap,that is not total junk. I bought most of this stuff when it was reasonable.
I even bought some of the Pakistan stuff to try out for $75 per can,I do not expect it to be very accurate,but it should all go bang.