.003" neck slop with the PMJ military brass and .3098" bullets. I can chamber these bullets with just the gas check in the neck, throat is .310 and a couple tenths when clean, acts like .3100" dirty, the bullets have to jump a long way to the throat, like .150" or so.
I have no way to tell for sure but don't think the bases were bumping up before the throat even with the softer bullets. A primer and 70% fill of rice grains is enough to get the gas check well into the throat and the PC is slick. I think the softer alloy was failing due to land engrave wear farther up the barrel. This seems to be the #1 accuracy killer with greased bullets at hv and I have a working theory (you'll hear more about this soon) based on lots of observations over the years that the torque stress accelerates drive side wear and even if it doesn't lead the barrel something about gas leaking around the bullet destroys accuracy. I can assure you it isn't casting defects or bullet balance problems because velocity limitations still occur when bullets are breech seated and it's worse/lower speed the more brittle the alloy, but add a paper or polymer jacket and fit the bullet to the throat and ordinary cast bullets can do amazing things without "RPM" limitations. Slower twists and lapped barrels help too, but the slow twist still has drawbacks in the accuracy department. Process of elimination tells me that whatever limits accurate velocity (seems to be related to twist rate and caliber) is happening between the front end of the throat and a few inches past the muzzle because we have ways of fixing everything else and the right alloy will get you farther than the wrong one id not using some kind of jacket.