kwg020,
I happened to make a somewhat inadvertent comparison of flash hole size a few month ago, when doing a lot of shooting with a CZ 527 in 6.5 Grendel, which typical of CZ rifles was quite accurate.
The first batch of brass I could find was Norma, which proved to be quite consistent in dimensions. Worked up a bunch of loads--and then came across a batch of Starline brass, so bought it too, which also proved to very consistent dimensionally, with one exception. It turned out the Norma brass had standard small primer flash holes, and the Starline had large rifle flash holes. (Both had small primer pockets.) All the flash holes were well centered.
So I retried three of the loads worked up with the Norma brass in Starline brass. There wasn't a consistent difference in accuracy--in two loads the Norma-brass loads shot more accurately, and in the other the Starline-brass load did.
The weight of both brands of brass averaged almost exactly the same. For 10 randomly selected case (all trimmed to the same length), the Norma brass averaged 111.1 grains, varying .6 grain. The Starline brass averaged 110.9 grains, varying .5 grain.
Despite the difference in flash hole size, muzzle velocity was for all practical purposes identical. Same deal as with the accuracy--in two loads the Norma brass averaged "faster," by 7 fps and 5 fps. In the third the Starline brass was 8 fps faster. This is less variation than the SAME load often chronographs in different strings during the same range session.
