"I call her Vera." This is Jayne's signature weapon from the Firefly series. I partly modeled it based on GameProps' design, but with higher detail: http://www.thingiverse.com/thing:690281.
Print one of each part unless they have a numbered suffix. "_2" means print two, and "_2M" means to print one normally and a second one mirrored. Use support or not as appropriate for your printer. You will need three segments of 0.5 inch PVC pie to act as skeleton.
Assemble sub-units, putty cracks, and sand. To each part apply a coat of glossy black primer. Metallic parts use a light coating of silver or copper metallic paint. For mixed-color parts, mask off the black areas and paint the exposed areas. Other metals can use shades of gray acrylic, and one or two parts need colored paint.
After painting parts the parts can be glued together for final assembly. It can take a wash of burnt umber to make it weathered.
Generally you'll need one of each part. The exceptions are part names that end in "_2" (print two of those) and "_2M" (print two, one as-is and the other mirrored). You will need three lengths of 1/2" PVC pipe -- one for the body overall, one for the magazine, and one for the scope. Rough-fit the parts and then cut to length. There's no detailed guide but much can be inferred from reference images, and many of the parts fit together like a puzzle.