The older frames are said to be made of cast iron or ductile iron which isn't as strong as a modern frame. Current production SB2 rifle frames are heat treated investment cast alloy, SB1 shotgun frames are weaker, and made differently, dunno exactly what they are, but the appearance is much different as can be seen in the FAQs comparison pics. H&R won't fit any high pressure barrel to any SB2 frame made before 1999, which would be an NN or HN frame, those barrels are marked with an asterisk in the accessory barrel list. As far as the old H&R Topper frames, they won't fit any barrels to any frame made prior to 1987. Modern barrels may or may not fit on the old H&R frames, I tried some barrels on a Topper 158 I had for a while, they wouldn't come close to fitting, but other members have been able to fit barrels to them.
The old H&Rs came in several different rifle chamberings, 22Jet, Hornet, 30-30, 357mag, 44mag, 45-70, to name some of them, all relatively low intensity rounds.
Also in the FAQs, there's a link to a post by a member who bought a 1940 H&R that someone had put a .223 Remington barrel on, it didn't have a happy ending for the firearm, but fortunately, the owner wasn't injured.
Tim