Once you do an OCW workup for each brand of brass you'll find out whether or not you can mix brands interchangeably.
Believe it or not I get very good performance from the same charge of 4064 in Lake City brass as I do in Winchester brass, no need to rezero the rifle. Of course 4064 is a "slow" powder for 308 and so it isn't totally burnt before the bullet is moving down the bore and increasing the effective chamber volume. Your experience with a different powder (or different brands of brass) may differ.
Machine guns are awesome until you have to carry one.