At least through 1934, Parkers were not proofed for more than 3" shells, p.515, TPS. Those had a working service average limit of 11,200 and were proofed at 16,800. A commercial 10ga 3 1/2" cartridge is at 11,000psi, so a sleeved gun should have withstood the load. Wonder if it was proofed with proof loads or just tested with regular duck loads.
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