I am in pursuit of a load for USPSA in a CZ 75B Omega. The bore slugs to .3562 and is a 1:10 twist. The pistol was shot in a Ransom Rest at 20 yards.
Everything is the same except the bullet size in the two loads. The brass was miscellaneous. The powder was 4.4 g. of Bullseye by a Dillon 550 powder measure. Primers were Winchester small pistol. Bullet was a my cast from a Lee 6 cavity 120 TC. They were powder coated by the shake and bake method using Smoke's black pc. The diameter was .358 to .361 after powder coated. I crushed one in a vise and the powder coat did not break down. The first 10 rounds were sized to .3570 thru a Star machine. The next 10 rounds were sized to .3582.
The .3570 rounds grouped something like 4" in a slightly horizontal group of 10 rounds. Nothing to get excited about.
The .3582 bullets had 7 rounds in a 6" group. There were 3 flyers that keyholed some 12-15" from the group center.
So the boolits that were .0008 oversized were stable but the boolits that were .0019" larger keyholed and in a larger group.
What causes a slightly larger bullet to tumble?
I shot other groups with different powders ( blue dot and AA no.7)where the .3582" boolits group better or equal to the .3570 but with BE they tumbled. Explain