ed,
Current English proof specs are higher than what was typically applied to older English guns. A few brands of guns from yore are gaining a reputation for failure at reproof, Coggies, among others, are in the group. But, and this is important, that failure is mostly the result of pushing the gun to a level of proof it wasn’t designed for, AND DOESN'T NEED. Just use or load the right damn ammunition. US Promo loads are some of the worst offenders. Quite a few of them hit over 10,000 psi, OK for your 1100 that hasn’t been cleaned this year. But, we don’t know what we don’t know, ed, and if you don’t have information demonstrating otherwise, it is better to assume your American 2 3/4” ammo is too hot for your vintage English gun.
The boxes of American ammunition printed with the symbol for CIP certification is a fairly recent development, likely related to manufacturers selling US loaded ammunition in Europe. For what it is worth, the French gun makers I hung out with, and drank too much at the evening meal with, were pretty much unconcerned with pressure from modern loads, but, were emphatic that an obstructed bore was a whole different deal.
Best,
Ted
