Any time a piece of brass is made a tiny bit differently (either because of tooling or a batch of raw product is different) the shooting characteristics of the finished product can vary a bit. I found that when I use LC .556 brass, (or other military brass I have used such as Winchester), I need to reduce my load by an average of 3/10 of a grain to match the same velocity with a given bullet, primer and powder - as opposed to using commercial brass, of which I normally use Nosler, Winchester, Remington and Lapua. I have never run into variations in LC brass (despite any extreme weight differences between lots) that created a need to change the powder by more than 3/10 or so of a grain to arrive at the same velocity between the lots of LC brass. This would tell me that I can load all mixed LC brass and, as long as it is charged 3/10th of a grain under the same load I'd use in commercial brass, It will be O.K. in respect to pressure, assuming the commercial load was not an unsafe load. The 3/10th grain variation within the LC brass (or LC versus commercial) usually equated to an approximate 30 fps to 45 fps difference when shot over a chrono. So - if you find an accuracy node that is very wide with a particular powder and bullet, the velocity spread may not even affect the accuracy as much as you might think. My own personal experience is that the differences between neck tension will actually make a bigger difference in accuracy than the brass weight differences. As such, turning the necks on your "mixed' brass to a uniform thickness will help to make them all shoot better as a group, though may defeat the purpose of mixing them together - if not wanting to weigh them. If you don't want to separate just because you don't have enough of each lot - turning necks is a great trick to make a bunch of odd lots work together better. So the short answer is - yes - mix them up if you want. For extreme accuracy, I'd sort by year, then weight and turn necks. For blasting - just load them all at least 1/2 grain below a commercial brass load that is at least 1 1/2 grains below the max - and you are good to go. I'd still work up from a lower load. Good luck!Can brass with different year head stamps be be mixed together in one group to reload. It is LC brass so why should the date it was made matter?