I'm going to toss another possibility out and beg for a chamber cast!

I wonder if this rifle is a 9.5x57, not 9.3. Spoon bolt handle and engraving just has me curious and leaning toward the 9.5x57 {or 9.3x62}. Such niceties are very rare on Swede 9.3x57's.

Granted, the larger bullet diameter {9.5/.375 vs 9.3/.366} would indicate lower pressures but maybe pressures themselves were not the problem. Easy seating would seem to rule out a 9x57, and 9.3x62 seems possible, but a loose-chambered 9.5x57 might have set up a similar condition of excessive headspace.

Does a 9.3 bullet fit in the muzzle or slide right in to the case neck?

Also, I forgot to add above, if the rifle is 9.3x57, you can use that 8x57 headspace gauge to good effect. You should get binding when you attempt to close the bolt on it. Remember to remove the cocking piece and extractor for best feel when using the gauge.