different brass
The thing that gives accuracy a problem is inconsistency. I was a fanatic about not mixing manufacturers. Especially when shooting Hunter Bench Rest.
Now, I mostly just shoot loads designed around hunting needs. I'm still an accuracy fanatic though. Once bitten, always addicted. I tend to go overboard as to my realistic needs.
I did extensive testing with an HBR in 308 Win with a Hart Barrel and 1-15 twist rate. The rifle was built by Nobby Uno and shot like a dream, as long as the bullets didn't weigh more than 150 grains. I positively loved Randy Robinette's 135 grain flat base spitzers.
I mixed up all sorts of brass to see if there was any real difference in accuracy out to 300 yds. The answer is "YES". There was a difference but not for the reasons most people think. If you've ever had anything to do with a bench rest or F class rifle you will understand that everything possible is done to remove variables from the rifles, then the cartridge case and bullets are next. Powders are bought in canisters of 7-8 pounds and bullets are bought in 1000 lots or more. Primers are usually some sort of bench rest type, usually picked for their consistent ignition.
When a lot of brass cases was bought, it was always sorted by weight. The batches were usually in 2 grain increments. Then, they had their primer holes deburred, drilled out to the same size. The batch of cases that contained the most cases was usually chosen as the competition lot. Then, the batches on either side were weighed again to find those within .5 grains of the control weights. The necks were pressed over a mandrel in a lathe and turned to give .002 in clearance in the chamber. Remember, this is all a controled condition match.
This isn't as bad as it sounds. It only needs to be done once and all of the sizing afterwards is neck sizing only on an arbor press, in Wilson neck dies with interchangeable collars. Because of the tight chambers tolerances, stretch is almost non existent.
The one thing that soon became obvious, is that cases from the same manufacturer varied in weight from lot to lot. Therefore, capacities were different. I found that as long as the cases wieghed the same, after neck turning, the internal capacities were close enough not to make any accuracy difference, no matter which brand of case was being used or mixed together.
The thing I found was that case capacity made a big enough difference to lose a match, not who manufactured the case. With an off the shelf hunting/varmit rifle, I seriously doubt there would be any noticeable difference as long as pressures were relatively consistent. After all is said and done, a consistent group is far superior when hunting than one which is a tack driver under conrtolled conditions. Temperature is also a killer. In my HBR rifles, I had special loads for different temps as well as high and low humidity. The humidity loads were mostly just anal thinking as it would only add or subtract 1/10 grain from the load being used.
