Well, I doubt they would cause catastrophic damage, H&R proofs them at pressures well over those cartridges, but the underlug is a sacrificial component, the case head thrust of those DG calibers would cause the barrel to be loose in not too many shots just as Handi35's .300WSM did, but his SB2 frame was just fine.
To give you an idea of their strength, one member while at the range had a friend fire a 7.62x39 round in his new .243 Handi, bullet went down range, but the pressure jammed the barrel in the frame so it couldn't be broke open, he sent it to H&R, they replaced the barrel at his cost, frame was undamaged, much to his satisfaction and surprise!!

Tim