I bought a Handi Rifle in .444 Marlin, not long ago (SB-2). Since the biggest reason for buying it, was to get additional barrels, I checked into availability and requirements before my purchase.
The end result is an action I can put any barrel on, except rimfires.
All four actions are castings.
The SS-1 is for rimfire barrels only.
The SB-1 is for pistol cartridges, the muzzle loader barrel, and most shotgun barrels; but not rimfires.
The SB-2 is for rifle cartridges, and can have any other barrel fitted, except rimfire barrels.
The SB-2-10 is used only for certain 12 ga rifled slug barrels and 10 ga barrels. Nothing else can be fitted to it (by H&R).
The .500 S&W is no longer built on the SB-1 frame. Too many of the SB-1 receivers got beaten up by the cartridge. Now, it is only offered on the SB-2 frame.
But... that only applies to current production. Older models can be found in other combinations. And, older models can still be found with ejectors for rimfire, pistol, and rifle cartridges; where newer production only has extractors, except for shotgun barrels.