Would be illegal in Canada to make one from a SxS or any shotgun.
If coming from a licensed manufacturer of firearms, barrel lengths as short as 12" are known (mare's leg m94), but the overall length must be 26".

For the citizen, a shotgun barrel or barrels can be shortened to a minimum of 18", however the gun must be over 26" overall length. So, if shortened to 18" and a pistol grip put on it, it would be a prohibited device/firearm if under 26" overall length.

Illegal to "make" a pistol out of a shotgun.

Used to be legal to make a muzzleloading or even a ctg. pistol, but of course had to be registered as it would be a restricted firearm.