I didn't have much luck with a unibit. Caught just as it was going through.
Much better luck with a rotacut sheet metal hole cutter. Back up the inside of the hole with a piece of plywood.
http://www.hougen.com/cutters/sheet-metal-hole-cutters/Rotacut-sheet-metal-hole-cutters.html
Knock out punch works great just a bit expensive if you only need one hole....
The SS might be too thin to punch without adding a layer of scrap to the area.
Bob Avery's Rotabroach tool is a delight to use for just this purpose. A little 'exe' but hurts only once.
1911A1 and a rat tail file.
would you suggest a wadcutter and how many grains?