Jim,
The Huntsman frame is a SB1, can have shotgun barrels fitted at H&R/NEF with the exceptions of 10ga and 12 Ultra Slug Hunter. Is considered a Firearm.
The Sidekick frame is shorter, cannot have other barrels fitted to it and is not considered a Firearm. It is strictly a muzzleloader and no 4473 form is required.
As far as shooting goes both muzzleloaders are almost identical except for the shorter receiver on the Sidekick. Hope that clears it up, if not let us know.