Take just a bullet and inert it in the muzzle to see how much play there is before it hits the dirt catcher band. I have a 358 Winchester I barreled up and normal 358 cast bullets are designed to bore ride on .350 top of lands. The top of my lands are .352 and show yaw thusly I am having a bullet mold made up with .353 bore ride which should alleviate the condition. I figured this out when I tried a bullet in muzzle and held it to one side and the yaw was clearly visible to the naked eye. Thusly when the pressure curve it, the bullet flopped to the side and stayer that way out the muzzle.
FWIW the Marine Corps rifle team used to shoot 168 Sierras at 1000 yards and won matches with their M14s with every round yawing as it went through target. They were stable to 900 yards and by the time it got to 1000 it had dropped below mach 1 but accuracy wasn't hurt so they kept shooting them.