One of the devs commented on reddit some time ago about the unreleased shotgun, that one of the reasons behind dropping it (IIRC) were that it couldn't be properly animated.

Community mods don't "require" polish to the same extent as an AAA game, and surely there are a lot of people who would love to see the KSG in the game. I suggest releasing dropped content as part of the samples in benefit of the community.