Now, if not, why not?
Just about any object can be used as a weapon. If the government were forbidden from taxing or placing any restrictions upon every item which could possibly used as a weapon, it would be unable to tax or restrict much of anything.
The variety of items which could potentially be useful as weapons in a well-functioning citizen army is quite large, and probably encompases most artifacts that would normally be described as "weapons". One might reasonably question, however, whether a cane gun could be effectively so used. Most such weapons are apt to be no more readily kept at hand than a pistol and are apt to be more difficult to fire either quickly or accurately. As such, their military usefulness may be dubious at best.
Something like a Glock 17 or M-16, however, would have clearly demonstrable military usefulness; something like a Bryco model 38, while not used by any military, could reasonably be allowed as a poor person's "nearest equivalent" to a good military pistol.