If you acquired U.S. (Lake City or LC YY) military once fired 7.62x51, then they were fired in a machine gun, with loose chambers and puffed out. All military 7.62 once fired is like that. First sizing tends to be very difficult to the extent you swear something is wrong. Go slow. Remove your expander ball from your die and only size the outside first. Good lube is critical. I use Lucas Bros. White Lithium grease, and it makes sizing these cases a lot easier. The primer crimp will need to be removed. I use a countersink drill bit and it just cuts away some of the offending brass. TRIM - your case lengths will be all over the place after sizing. Some will need possibly zero trim, while others will need 15+ thousands taken off. Personally, I prefer to buy this brass pre-processed to the full extent possible. It is worth it, unless you have the equipment and too much time on your hands. Weighing the cases, and separating into like groups is supposed to aid accuracy.
After you process the brass, all charges for 308 in 7.62 military brass should be reduced by about 2 grains, in addition to reducing 10% for your starting loads. This is
true for 7.62 which do have thicker brass. It is NOT true for 5.56/223 from LC which runs right on par with commercial 223.