There are a number of variety of cane guns. Some antiques, some NFAs.
First, to be an antique firearm, it must have been made prior to 1-1-1899. That is, not that model, but THE GUN had to have left the maker's bench prior to that date. Weapons that use loose ball and powder are also antiques, without regard to date made.
Let's say you have a Remington Cane Gun- in caliber 32 S&W. That falls under the NFA- National Firearms Act- as an AOW- Any Other Weapon- since it is a disguised firearm. Must be registered with the BATFE, and a $5 transfer tax paid.
IF NOT ALREADY REGISTERED- it cannot be registered. It must be surrendered to the BATFE for destruction. Possession of an unregistered AOW carries the same penalty as possing an unregistered machine gun.
What we have here is... failure- to communicate.