The Gabbett-Fairfax Mars, just to see if it was the monster it was reported to be. Though I'm sure an autoloader in the equivalent of .44 Magnum was quite impressive in an era where .30 or .32 caliber was considered "enough gun."
I'd also want to shoot the GAU-8/A 30mm Gatling gun, as packaged in its "A-10 Thunderbolt II" transportation system, otherwise known as the "Warthog." And probably as "OH SH...." to anyone on the receiving end.
Finally, one of the chicken guns, just for grins. Any one of them would do.