Large flash holes
One of the most fun things we did, a few years back, was drill .44 mag brass flash holes to 1/8", prime with CCI 350, and load with parafin wax. Shooting through a T/C Contender, no powder...it would shoot groups the size of a quarter at 50', and penetrate 5 layers of cardboard. It sounded like a cap gun, and was deadly on sparrows, rats, etc. To load, you size the brass, warm the parafin slightly, so its not brittle, then press the case through the parafin. Prime them, and you're ready to go.
English sparrows like to roost on sliding track doors. With this rig, we'd aim a hair low, and the wax bullet would shatter, wiping out the sparrow, and lubricating the door track. A win/win.
With the Contender, when done, pop off the barrel and a quick scrub took care of everything. Now...why did I ever trade that thing off?