I went from 0.318" bullets to 0.323" bullets in my Gew88/05 Commission rifle. The 0.318" bullets were just causing problems.
Here are some pictures of primers. The brass is 7.92x57mmJS (8mm Mauser) formed from Winchester .30-06 brass. Bullets are 150 grain Hornady. Powder charge is 43.5 grains of Varget, and primers are Federal Large Rifle. Published velocity for this load is 2400fps, which is just fine. Figure it might be a bit more with the tight bore.
Two different exposures with their inverted images on the right side
The primers look like they're fine to me. Do you agree?
Data is from Hornady and the 43.5 grains Varget is their starting load. Hodgdon's starting load is 45.5 grains. Figure I'm safe using 43.5 grains as a start and 46 grains as max -- do you agree?
Also, this is effectively a squeeze bore. The initial slugging of the bore resulted in a groove measurement of a little over 0.318", but subsequent cleaning and shooting have been peeling off grime to reveal something a bit bigger. I still need to slug it again, but it may end up being 0.321" as they were playing with sizes of 0.318" to 0.321" while developing the rifle, and I have an original 1892 barrel. For now I'm assuming a groove diameter of 0.318" to be safe.
Would you have an estimate to what kind of velocity the barrel adds if it's 0.321"? I'm not trying to push the rifle's limits at all; just curious. I know there were squeeze bore barrels made for the .22LR before the advent of the rimfire .17 calibers, and they boosted by a few hundred feet per second.
Subjectively, recoil is about like a .30-30 with 150 grain load, maybe a bit less. Feels good to shoot and if I can find an accuracy node someplace under 46 grains Varget, I'll be a very happy camper.