Help me out here.

How important is using the same head-stamps for accuracy.

If you put the same load/bullet in 20 federal, 20 Winchester and 20 Hornady casings how much of an accuracy difference might there be between the same head stamps?

Times are tough and I may not be able to get 50-100 of the exact same brand casings.

I can shoot sub moa at 100 using mixed headstamp. I tested this with 5 different cases in a magazine and was able to shoot it consistently. When shooting at 500 or 600 yards, it may be different. But my shooting is generally at 1-300 and I have zero issues.

If you are looking for really tight groups at really long ranges,  not only are headstands important, but so are lot numbers.  Using the same lot number as in buying bulk of many cases from the same lot number theoretically ensures a consistent load and powder charge.  If you are shooting at 300 yards or less, the lot number and head stamp probably doesn’t matter.  In today’s current market, buying multiple cases within the same lot number may not be possible.

Appreciate the info.

I should have mentioned this is for eastern hardwoods whitetail  hunting. 200-300 yards would be max for sure.

Since it is also not stated, what caliber are you using?  If it’s .223, that’s not the best caliber for deer hunting.  If you are using .223, shot placement becomes even more important.  While this may go without saying, please don’t go deer hunting with FMJ ammo.  You’ll need hollow point, soft point, or ballistic tip for that task.

<This.

.308 brass is not mixable IME. Lake City 7.62x51mm weighs 20 to 25 grains more than Winchester brand commercial .308 cases. .223 is much better in that regard.

I was focusing on 7mm Rem mag and 308 Winchester.

You really think a load under or at max listed grns would be a safety hazard from one brand of brass to another?

This also wouldn’t be a complete mix. More like 100 casings of 3 different brands. So it would be batches of 30ish for each brand.

You can use different brass, but it’s best to work up a load for each type.

For a .308, I had a charge in Lapua brass that was .3grs less than when using FGMM brass. For Winchester brass, the required charge as .4gr more than FGMM. If you develop a charge in Winchester (for my example), if you used brass that was thicker and held less capacity, you would be .7gr over the desired charge, and possibly over pressure.

YMMV, but I wouldn’t chance just running a standard charge in all cases, especially for accuracy.

Yes there are huge capacity variations in certain calibers in general and certain brands specifically.

As has been stated .308 commercial brass has way more capacity and will tolerate higher charges than thicker 7.62x51 NATO brass without showing pressure signs.

This seems counter intuitive, but the thicker brass causes more pressure for a given charge.

When talking about brands in particular I have found PPU brass to be much thicker than U.S. made commercial brass in almost every caliber.  This causes significant over pressure if PPU brass is used with data worked up in Winchester or Remington cases.

Most published starting loads are maximums in Lake City 7.62x51mm brass. The only exception being Hornady's data. Know that prior to testing.

The most comprehensive load data available for Lake City 7.62x51mm brass was published by the NRA in their now out-of-print book, "Semi-Auto Rifles - Data & Comment". It's available used at online book stores. No other data source comes close.

I load bulk with mixed brass, and it generally falls under 3moa.

Never tried with a quality bullet.

Assuming a safe for all charge is found, with good prep and match bullets, you're liable to have multiple distinct groups, or one large group at close range.

E.g. yesterday I had some "seconds" 165gr .308s go into an overall 6" group at 200yds. There were 3 sub groups forming, closer to 1½moa each, from the different headstamps. I don't care enough to go further into it, so I accept that 3moa is good enough for bulk.

For hunting ammo, this is not good enough. Pick one headstamp and work up a load for it. If you want to hunt out to 300yds, you'll need to get below 2moa (6" @ 300yds) to ensure good hits. Easily attainable with half decent practices and components.

Just sort out a couple boxes worth of one headstamp and go to town. For sporting ammo, it won't matter which one you choose, just that they're all the same.

You may find that multiple different ones weigh the same, with further testing you may find this to result in similar velocity/pressure, but may still have a wildly different POI at distance.

Please, verify your sporting ammo at 200yds or further. Lots of things show up at the further distances that are not readily apparent at 100yds. I don't want to hear a story of your tracking a wounded deer for hours to find nothing, nor do I want to hear about how you missed. We want to see you holding up a trophy with a center-punched heart.

Unless you're hunting near me, then I want you to miss.

This. It may, or may not, impact your accuracy. It depends how you define accuracy. Also, if you are loading at the very top of the pressure range mixed headstamps is a bad idea. For casual shooting with moderate loads most people wouldn’t notice a difference. If you want to get in the top 10 of the MOA Challenge you will want your cases sorted and fully prepped.

For .223 I find there are basically two specs of brass:  US based, and European based, is how I call them.

And within each spec, for all practical purposes, they are interchangeable.  But if you mix them, then a GECO  in a pile of Win brass is going to shoot differently, particularly at distances.  But if I run LC/PMC/IMI - they all shoot about the same.

And then you can start segregating by run out variation in the neck wall.

The less variation in case dimensions you can achieve the better.

My Varmint rifle barrels (.22-250AI and 6 mm REM AI both have tight necks.
Unturned brass will NOT fit in the chamber.

That was a condition my gunsmith insisted on.
If you want it tight necked, make is so that a factory round cannot enter.
I turn the brass for both rounds to have a neck wall thickness of 0.0100 inch.