These (my Moxxi and my hubby's Dr Zed) are my first cosplays, but being the perfectionist that I am, I find that with enough research (and enough dicking around), just about anything is possible. Even if there isn't a tutorial out there for Rubi yet, there are plenty of tutorials for other guns, so I'll wing it between that and instinct
Like PetiteAltesse said, you can always go the Nerf gun route if you don't want to get too crazy into sculpting and whatnot, and there are quite a few tutorials on adding electronics to props if you look for them. If you end up deciding not to put electronics in, that works too, people will recognize your costume and love your work even if it's not 100% accurate, but if you have the time and resources to research and experiment a little, you may surprise yourself with what you come up with
I'm sure you'll do a fine job on your props, no matter what you choose to do!