There is a little more wiggle room for the bullet in the 300BLK magazine. These are 150 grain 300BLK cartridges in the photo (my shoulder not strong enough to pull out the 300 BLK suppressed ammo yet).
Think I will also color code the “PMAG” logo on the Gen M2 (white) and Gen M3 (green) dust covers so I can keep them straight - yes I do use the dust covers 
I know this is not the most interesting thread, but … here we are! 