Addressing your original question, Gabe:
What do you think of this idea, is it legal, practical, shootable?
1.
Legal: No, not without jumping through the hoops to register it as a Short Barrel Firearm. National Firearms Act of 1934 (as modified) requires rifles to have 16-inch barrel
and 26-inch overall length, unless you do the registration thing. Shotguns must have 18-inch barrel(s) but same 26-inch OAL.
2.
Practical: Depends on definition. This also relates to the third point, below. It is heavier, longer, and less convenient to carry than a long barrel .44, .454, or .480 revolver. For
most users, the revolver is rather more practical, not to mention the fact that it is usually less expensive, considering cost of acquisition, tax stamps, and gunsmithing. On the other hand, with many loads, higher velocities are available from the 16-inch barrel. If dead-serious, massively
POWERFUL bear protection were needed, such a firearm in .45-70 or .450 Marlin might be an option.
3.
Shootable: See 2, above. Unless you get into the "spinlever" action or other contortions, you need two hands to operate a lever gun on second and subsequent shots. Also, one is hard put to get any accuracy, shooting one-handed with such a nose-heavy firearm.
All in all, the same general type arm, but a touch longer, is readily available, from both Marlin and Winchester. The standard length stock makes it MUCH easier to shoot well. I muse that one of these, with a tax-stamped ten or twelve inch barrel, would be very handy. You wouldn't really be giving up MUCH velocity, with the .44 Mag cartridge, as it was developed for revolver length barrels from the start. Again, for SERIOUS power, same SBR in .45-70 or .450 Marlin might approach optimum bear protection . . . .
My two cents measure, and worth every penny!
Johnny