Bob,
That " confusion & misunderstanding" is exactly why I corrected your comment from "Rifle" to "Pistol". I offer that most newbies, know a 357 and a 44 is a pistol/revolver round because they where developed originally for a handgun. Saying "rifle" to such a person denotes 308-30/06 etc. These calibers have never been offered on a SB1 and should never be put on one.
NOT all "pistol" calibers are on a SB1 either! The 45 Colt comes on a SB2 as does the 500 S&W as you mentioned. Only recently the 357 and 44 where demoted to the SB1 receiver as well...
Now moving from handi calibers for a moment, and in referance to the no pistol calibers comment. I offer that there are many calibers designed for handguns that have not been factory chambered in a rifle... 25 ACP, 32 ACP, 32 Colt, 32 S&W, 327 Federal, 30 Herrett, 38 S&W, 357 Herrett, 10MM, 45 AR...to mention but a few... I think you see my point. These are handgun Calibers. Making a comment that there is no such thing as pistol caliber, only "calibers and rifle calibers" is simply incorrect and misleading. Especially in light of your next paragraph. Where you comment about a newbie misunderstanding an obvious (To most of us) handgun caliber like 357 and 44 magnum to me seems queer.
Also in reference to the accessory barrel program, the factory will fit 99% of there barrels to the SB2. But only shotgun, 357 Mag, 44 Mag and a BP barrel to a SB1 frame. (The two "problem" barrels are the 10Ga and 12 Ga USH.) Even then, as mentioned there are small firing pin and large firing pin SB1 receivers out there. Large FP can cause problems with the 357 and 44...
Another frame I did not mention initially is the SIDEKICK. It was created to get around the firearms act of 1968 and skirt the requirement to fill out the form 4473. As this receiver was and only is used for black powder firearms and will not fit rifle or pistol or shotgun barrels.
CW