So underbarrel weapon says that generally, assign a +4 concealability penalty to them. That is great for flamer throwers under your assualt rifles, rocket launchers to go with your autocannons, etc.

But what do you think of the more optional "underbarrel" weapons? For example the over/under (or side by side) single shot shotguns (or elephant guns). Or over/under single shot heavy pistols. Or swords with a pistol incorporated into them.

Do you think they should have a modifier attached to them? The pistol I can see, because it is bigger, but not +4 bigger. +1 or +2 probably. But with the shotgun or sword, it's really not making the weapon much larger a whole lot larger.

Well, an AR already has a pretty piss poor concealability, but if it matters, would say that a two-shot shotgun or a heavy pistol would add a +2 modifier.

At that point it's largely academic. You've gone from "Everybody on the street knows you have a tricked out assault rifle under your coat" to "A quarter of the people don't see it...until everyone else points it out".