[Low Tech] Martian Shield Gun?

[Proteus]

The Space:1889 steampunk RPG (by assortedly GDW, PEG, and Uhrwerk Verlag) has a Martian city-state called Oenotria, whose crack troops are famous for their use of "shield guns."

You can find pictures of a couple of wargames minis at either of these pages, or a good look from behind the figure in this auction listing.

How would you stat such a thing in GURPS, and what tactics would it typically use? I'm guessing it's a blunderbuss, both because of the relatively primitive Martian technology and the inability to sight well around the shield. Would the shield be much good in melee at all, or is it just to protect from return fire, and then be discarded when action closes to hand-to-hand?

EDIT: And here is a page which (a ways down) has what looks like some of the original line art depicting the weapon.