Actual strength is another mystery, there's obviously less material there, but the heat treating and alloy used isn't known, H&R has never shared that info with anyone other than what's been said here, basically other than SB1 = ductile iron = cast iron, and SB2 = heat treated alloy steel.
I bought my USH for a rifle project, but after seeing it and the thin frame sides, and realizing the firing pin issue, decided it wasn't worth the expense of making a barrel for it, the 6.5mm Shilen match blank was just right to have it turned down slightly to fit the SB2 USH frame, but I ended up having a stubbed 6.5x55 done on an SB2 rifle frame instead.
The USH is now the platform for the 22 Hornet adapter Larry made, talk about a BULL barrel!!
Tim