I've tried a few more weapons. Not sure that I like them. I still like the Marauder gun. Have a few coming in the mail.
The first is the barrel from the Jungle Assault Ripcord fig. It's okay, but it's a little too slim and you can see the ball popping through the top, which I guess isn't a big deal.
The 3rd pic is a Star Wars gun, one of those extras that comes in the little box with certain figs. The right side is just the barrel while the left is the whole gun without the handle. I like just the barrel one better, but again, it's just okay.
The last is the gun from the JUMP jetpack. I'm not really diggin' this one. Of course these all need to be clean up a little, but it's just some rough ideas. Like I said, I like the High Energy Plasma Disrupter better, and that will probably be my default.