So here's what I have managed so far.
The raw materials are just a Martian Imperial Guard Halberdier - one with the halberd held close to horizontal should work best - plus a pavise from the Imperial Guard with guns.
I trimmed the halberd a few mm forward of the left hand, to allow the "shotgun" to project through the shield a bit. I also bent the end of the halberd down to suggest a shotgun stock. I then chopped and filed the pavise down a bit, and drilled out a hole slightly off-centre to take the shotgun.
And that's pretty well it. Between the left forearm and the "barrel" you've got a pretty good surface area to glue the shield on, and here are three based up. I think that the notch in the pavise/shield works nicely for the look.
Not too bad for 5 minutes work really, although I may add a shield strap if I'm feeling adventurous. I also have to decide out how to deploy them. I may mix them with my halberdiers, as shield guns are essentially close combat weapons.
