The first shotgun I ever shot was Dad's old Eastern Arms singleshot, which had been rechambered from 2 1/2" to 3". Sounds risky: 1/2" longer shell, higher pressure in the .410. But .410 chambers are particularly beefy, so unless you have one that tapers down sharply at 2 1/2", it's not likely you'll get a blown chamber. A.P. Curtis, who wrote a couple articles on short chambers for The American Rifleman in the 30's, reported firing a lot of 3" shells in 2 1/2" chambered .410's. Other than blown ends on the shells, he reported no issues--although he did suggest looking down the barrel to make sure that a piece of the hull didn't lodge in the bore. He reported improved patterns over a 3" shell in the appropriate length chamber, but concluded that the same pattern improvement could be achieved with a shell only 1/4" longer than the chamber.
