shotgun scabbards in my opinion are this goofy new fad that people are trying to get in on because it looks "tactical," when in truth, it's just another piece of gear to weigh you down and crowd your flak (or whatever MOLLE you're attaching it to). if you're going to use a shotgun as a "secondary," then it should be compact compared to your primary long arm since its main purpose isn't to be your workhorse weapon. range should take a secondary consideration to accuracy and reliability, and this weapon should really only be a bridge to getting your primary back up and running (as any secondary weapon should).
airsoft, unlike real life) doesn't have any secondaries that really offer a completely different capability that the primary doesn't (save for grenade launchers that shoot bb showers or other projectiles). sure, methods of operations will vary, but ultimately they all shoot the same 6mm bb. unless you really want a tri-shot, then i'll have to say to hunt down a Maruzen CA870 and enjoy. who knows, you may just love it more than your primary.