The issue with trying to implicate the tag to handgun is that pepper-box describes the mechanism of the gun alone and not which kind of gun it is, whether handgun or long gun. The wikipedia article itself has a photo of a pepper-box carbine showing this to be the case. It's why I don't believe the revolver tag should have an implication to handgun either, as things like post #4126295 exists.
As for the implication to revolver, I really think visually the pepper-box is visually distinct enough from a normal revolver to warrant not lumping them together.
Unfortunately even if pistol is incorrect, it will end up a common mistag due to the fact that in common usage the word pistol is used interchangeable with the word for handgun. You can see this currently with the revolver tag, see revolver pistol search. An implication won't resolve this issue because of this.