I think the likely problem comes as a result of recoil, and pressure isn't a significant factor in recoil. Velocity and shot charge determine recoil. I've owned modern Webley and Scott 20's made for export to this country. Often weighing less than 6 pounds, and some of them with 3" chambers. My concern shooting an American 3" 20ga in those guns wouldn't be pressure, but rather how the recoil would impact me, on the back end of the gun. And what it might do to the stock. I've looked at those guns and thought to myself: "Someone might be crazy enough to shoot magnums in them . . . but not me!" I'd be somewhat hesistant over standard 1 oz 20ga target loads, like Winchester's AA Heavy Target Load. I wouldn't worry about standard 7/8 oz skeet loads. But as noted above, Claybuster's 3/4 oz wad makes loading light shot charges a piece of cake. And they break skeet targets just fine.