2 1/2" shells, as far as I know, are all CIP approved. In 12ga, that means a maximum service pressure of 10,730 psi compared to our American SAAMI standard of 11,500.
But here's the rest of the story: Many of the foreign lead loads available in this country (when they're available!) are hunting vs target loads. Heavier shot charges and more velocity (meaning more recoil) than you may want, particularly for target shooting. RST's 2 1/2" shells, as well as being low pressure, are mostly modest velocity and lighter shot charges. So lower recoil.
Nontoxic loads for vintage guns are another topic. British shotshell makers offer steel loads which are approved for use in "standard proof" (previously marked 850 bar, now marked STD) shotguns. But they have less velocity and are restricted to smaller shot sizes than are steel loads for guns that have passed steel shot proof. And I don't know if they're available in the States (or will be when the supply chain is actually working). If I were looking for nontox loads to use in vintage guns, I'd probably look at Kent's Tungsten Matrix loads. I have a few boxes of those in case I choose to use my 120 year old William Evans for pheasants where nontox is required. They're as good as lead. Shot charge is relatively light (1 1/16 oz), which is OK for me as long as I don't try really long shots.
I've got a bunch of TM in both 12 and 16 but it's all heavier waterfowl loads. Nothing I want to shoot out of an old gun. A few years ago I pulled apart a bunch of TM #6 and a friend reloaded the shot into some nice 1 oz low pressure loads. I'm just not shooting at ducks enough.
