I'd say we've gone too far in both directions in this thread.
Yes, the NID is a strong gun. That being said, "modern" in the mid-1920's does not equal "modern" 85 years later. The Winchester Super-X caused Ithaca to switch to a stronger design. Someone here (with old Ithaca catalogs)--or maybe Walt--would probably know. If Ithaca went to 2 3/4" chambers, standard, when the gun first appeared--in 12ga--then all NID 12's should be able to handle anything up to and including a 3 1/4 DE-1 1/4 oz load, which is essentially what the old Super-X 12 was.
However, I know for a fact that the NID 16's, and I think the 20's as well, did not originally appear with 2 3/4" chambers. And the old 2 1/2" 20's and 2 9/16" 16's were not the equivalents, in terms of pressure, of some modern 2 3/4" shells. I'm sure a lot of NID's have held up shooting modern factory shells, even if they started life with short chambers. But it's not something I'd recommend.
And Joe, if you're shooting American factory 2 3/4" field loads in a 1925 Scott, you're almost certainly subjecting it to pressures for which it was not designed--even if it's a waterfowl or pigeon gun. The current standard CIP proof of 850 bars equates to a service pressure about 1,000 psi lower than the modern SAAMI standard. You could well have been lucky, but you're not doing your gun any favors.
