Your rifle has excessive headspace. Allowable headspace is .006" to .007" only by SAAMI and CIP standards. The primer cannot backout more than that amount in a factory rifle. Perhaps your bolt or action lugs are set back?

The heads should not have expanded with such light pressure loads. I cannot fathom that, except that perhaps your measurements are catching the web expansion, not just the solid portion of the case head.

When measuring head expansion, it's best to mark the rim in two spots (felt pen) and measure that on a new case, before and after firing. That is the case head expansion measurement we're looking for, not on the case with a round anvil, ahead of the groove. If your mic, had blades instead of anvils, it could more accurately measure actual head expansion, in the grooves or just at the top edge of the groove. It's just easier to measure the rim- but mark it. The spot of measuring must be marked so the measurement can be taken at exactly the same spot each time.

Because the primers backed out, shows your actual pressure was less than around 35,000CUP- probalby WAYY under due to the horrid velocities noted. Primers back out only due to headspace, not excess pressure. At about 38,000CUP, the case will lose it's ability to withstand the building pressure exerted against the walls of the chamber and the case will slide back in the chamber to be stopped by the bolt face. This sliding back will re-seat the primer to flush with the case rim. That is normal and what happens to most factory ammo today - expect for the VERY lightly loaded ones, like the 9.3x57Norma which runs aorund 40,000PSI. The same rifle is chambered for the 6.5x55 which generates 55,000PSI & the 9.3x62 which generates 57,000PSI.

Not sure why I thought 6.5x55 brass might work.

What is the ac tual measurement on the expanded case web?

The 6.5 X 54 MS is rated at 53,000PSI or 46,000CUP. The 6.5x54 Mauser is rated lower, at 39,000CUP or 44,000PSI - all CIP (European law) measurements. I surmise that is why factory ammo delivers some 2,200 to 2,330fps or so, with 156gr. bullets.

I would think 2,200fps would be a worthwile goal, IF the rifle didn't have excessive headspace.