Anyone with any serious knowledge of SB2 rifles knows the underlug will fail long before the frame does, it has a built in relief feature. I call BS on anyone that says they've seen an SB2 frame fail catastrophically. Internal flex, yes, it's common with hot loads in high pressure chamberings, but not frame failures.
A very good example of the strength of the frame itself is a (ex)friend of member here accidentally loaded a 7.62x39 round in his 243 at the range and fired it, it locked the action up, so he contacted H&R, he sent it in, the frame was undamaged, they removed the barrel with the damaged underlug and refitted the frame with a new 204 Ruger barrel and returned it to the owner. I don't know what kind of pressure would be created pushing a .311" bullet down a .243" bore, but it's got to be considerable.
Modern SB2 frames are proofed at 70kpsi, you may grenade a cast iron frame, but not a modern SB2, comparing any cast iron frame to a heat treated investment cast alloy steel SB2 frame is ignorant.
Tim