John, the Versapack frame is an SB1 frame as you can see in the rimfire sticky, dunno it's strength, but it's a moot point as far as a factory fitted barrels other than the original 410 or offset bore 22Lr barrels, H&R won't fit ANY other barrels to it, even tho members here have tested other shotgun barrels on it. As Keith said, frame strength isn't really a question tho as far as a muzzleloader barrel is concerned. The only way to know if it will work is try a used one.
Since H&R will fit Huntsman barrels to SB1 frames, is should work fine, they just won't put one on for ya.....for whatever reason.
Sidekick frames are smaller and NO other barrels will fit it.
Tim