There's another little trick I use a lot, since I have thousands of empty brass cartridges around for reloading. A .30-06 case sharpened by beveling the case mouth with a pocketknife is a perfect size for cutting out 1/4" bolt holes in gaskets, and a .38 Special case works well for 5/16" bolts. .45 Auto or .45 Colt cases work well for 3/8" and 7/16". If you've got a .50 BMG case around, it works for 1/2" bolts nicely! Don't hammer on the cases, just push down and twist them over the spot you want to cut for the bolt hole.
Jerry
