RR, if the gun came from the factory with 2 3/4" chambers--and I believe, from previous discussions, Ithaca had gone to 2 3/4" in the 16 by 1937--the ammo available then was essentially the same as the ammo available now. SAAMI standards for the old, 2 9/16" 16ga loads, on the other hand, were about 1,000 psi lower than for 2 3/4".