The #3 frame Parker action shouldn't be any problem. Remington routinely rebarrelled Damascus era actions with 3 1/2" chamber ten gauge barrel sets. Frank Lefever and Son sleeved the same old actions for 3 1/2" ten gauge shells. I measured my Lefever-Parker ten gauge magnum and it measures about .100 at the 3 1/2" mark. I also measured a lightweight Damascus Parker ten and it measures about .100 at the 2 7/8" mark. If we were to measure a factory 3 1/2" chamber Remington gun, I would bet that the measurement wouldn't be far from .100. I would be shooting your gun with 3 1/2" shells without much worry. Compare the outside diameter of the barrel at 2 7/8" and 3 1/2" from the breech to get an idea how much wall thickness was given up when the gun was rechambered. I'm guessing it isn't much.
