How much difference does it make in accuracy if you mix-match brass? Like Winchester,Hornady,Frontier, and Rem.I would like to know if this might have something to do with my group.

Well, mixing brands of brass is just something that is not a good idea. There ARE differences in case capacities of different brands of brass.

For example, using Winchester brass in a 22-250 WILL increase pressures as compared to Federal or Remington brass. The Winchester is good brass, darned good, but it has less capacity than the others, it demands a reduction in powder when working at maximum pressures.

Similarly, using Remington brass in a 300 Weatherby will increase pressures for sure compared to loads using Weatherby brass, period!

In some calibers, the difference is pretty much negligable.
But why chance it? 7mm-08 brass is not that expensive nor that hard to find.

Actually, when working up a load for a hunting rifle in 7-08 I doubt that switching brands of brass will show up all that much on your groups. BUT, when working up a load for a target rifle for silhouette shooting or some such use, I would not consider mixing brands of brass.

If you have several brands of brass you might consider using one brand for practice or varmint loads and another for big game shooting. I would recommend you do not mix them for any given use.

For example, I use Remington brass in my 25-06 for my varmint loads. I use Winchester brass for my big game loads. This makes it easy to see which loads are which when they are loaded with similar types of bullets. (Say 100 grain Nosler Ballistic Tips vs 115 grain Nosler Ballistic Tips.)

R F

THANKS FOR THE REPLY. I HAD SEEN A DIFFERENCE IN THE CASES WHEN I PUT IN THE POWDER SOME OF THEM WERE RIGHT AT THE NECK AND SOME WASN'T SO THANKS AGAIN FOR YOUR REPLY.

accuracy will suffer. Load up some ammo, same charge, primer, bullet, etc and shoot some groups with the ammo segregated by headstamp and then shoot some groups using random rounds drawn blindly from a paper sack and you will answer you own question. It may or may not be enough to be significant in a hunting situation but, I doubt, you will shoot any wall hanging groups with the randomly drawn rounds.
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It won't matter if you're willing to verify capacity of each case, but that's a pain in the drain. Granted, there can be differences in case capacity from the same manufacturer, but weighing cases for sorting is a lot less work. It's just a lot easier to stay within the same brand, whichever that is. If you pick up brass at the range, eventually you'll acquire enough for separate groups to reload.