Just fiddling around...
Since I was curious, you can indeed, with a lot of bullshitting, build an over / under. Sadly there's no "huge, stupid computerised scope" part yet, so I made one up.
And this is what people mean when they talk about a Beretta M92FS.
Also, given the greyed-out parts in the "select a preset" menu, it looks like he's planning to add a 1911 later. Milkor MGL would be a nice addition, as would USAS-12 and some ridiculously oversized NV scopes. Placable screws and bolts in a few size presets would be kinda neat, too.