I use a small screw gun with hex ended drill bits. This will fit inside a 12" tom.

Lay the shell on a piece of wood and drill it from the inside out. Its very quick, easy and very effective.

I use a 13-step UniBit step-drill, because it cuts clean, won't grab the material, has most sizes built-in, and there is a small chamfer between each step that you can use to your advantage to deburr the drilled hole. I don't use any twist drills to drill holes in pearl or shells. But the tapered reamer is also an old standby, and highly recommended if you don't have a UniBit.

You wont get burrs if you drill through the hole into a piece of wood against the shell.. I have step drills and tapered burr bits also I use on some things. I use a carbide burr bit to remove vent grommets with a angle die grinder..