>6 marlin 60s
>remove trigger mechanisms, magazines, and stocks, sell
>cut bolts in half
>weld firing pins to back halves of bolts in the forward position
>cut some threaded holes into the bolt halves, add pin things
>cut a slot in the receivers opposite the ejection port
>figure out someway to spring-load the back halfbolt
>make gatling core with grooves that pull both bolt halves back, lets the front snap forward, then lets the back snap forward
>3/8" copper refrigerator tubing coils around the barrels, feeding into the 'mag ports', wrapping back behind the actions, to a loading port
>air pump
>Lots more 3/8 copper in coils
>600 round bursts, pneumatically charged
Obviously there's more to it than that but it's a start.