[safisher]

Supposedly, there was such a thing as a Mershon & Hollingsworth Revolving Cylinder Automatic Rifle. Get one of those. Convert to cartridge. Or, set it up to pop the whole cylinders out and back in, like a speed-loader. Many revolvers worked that way in the percussion cap days. They weren't very fast, but I suspect you aren't looking for realism. Charge a Perk for "Early Adopter" and you are done.