Your situation can really be a head scratcher. I’ve only had one rifle that developed pressure early, but it was a good experience since figuring it out was such a head scratcher and forced me to look closely at every possible cause. I say “good” experience now, but at the time it drove me nuts and became a group project with friends helping to tract down the issue.
The sticking might be pressure, or might just be the relatively straight case and normal pressure wanting to stick a bit. The chamber might be extra rough, or just more straight than it would be in a perfect world.
Ejectors with a sharp edge can show a ring at low pressure and some brass lots are simply softer than others. Pushing the shoulder back more than needed can give the cartridge head some running room to slam into the bolt face, and an extra smooth or oily chamber can contribute to bolt face pressure. Unscrewing the sizing die a complete turn to avoid any shoulder set back will rule out excessive headspace causing marks at low pressure.
CCI 250 is a fairly hard primer and doesn’t normally flatten out around the edges unless there’s decent pressures. However, excessive headspace can allow the primer to stick out away from the case head early in combustion, then as the case head stretches to meet up with the bolt face it pushes the primer back in under pressure and it looks more flat than it normally would.
My early pressure rifle made purchasing a chronograph a good idea to see if what’s coming out the muzzle matches the powder charge. It helped, but the rifle still had somewhat low velocities at high pressure and somewhat of a low powder charge, but it was a valuable clue nonetheless.
It’s rare, but some powder lots are slow. I changed lots with no change in velocity or pressure.
Carbon rings can drive up pressure (but also velocity) so I scrubbed the bore clean, with no change.
I wanted to see what was going on with the case head, and learned about measuring case head expansion with a .0001” micrometer, and counting the number of reloads before the case head starts to thin. Cases lasted between 7 and 10 reloads, before case heads started to thin or case necks split, and primer pockets were still tight, but case head expansion was right up there with other max loads.
By every measurement the gun was reaching max pressure, but at a lower than normal powder charge and the velocity was about 150 fps slower than a typical rifle in the same cartridge. Cases fired a number of times with neck sizing only showed the chamber wasn’t tight, but very loose, which would normally reduce pressures. In the end I learned not all bores are the same diameter - some are a few .0001” big and some are tight - mine was tight, which drove up pressures early and may have been the reason the rifle had a oversized chamber. If pressures were too high during test firing with a normal chamber, enlarging the chamber with a slightly larger reamer might reduce pressure slightly and allow the rifle to be close enough to ship.
I don’t miss that rifle one bit. Lol
