Fixes freaky shotgun voodoo.

Currently you can saw down either of the pump-action shotguns and it
will mystically transform them into a sawed-off double barrel shotgun.
The simplest solution to this weirdness seems to be simply limiting the
recipe to only use a double-barrel shotgun to begin with.
Merge pull request #8081 from Rivet-the-Zombie/shotgun-transmogrification-fix

Fixes freaky shotgun voodoo.