Hi Idice,
A very good way to cut such is with a sharp cork borer. This type of tool was used by chemists, and still is in some cases as I still use one from time to time, to cut nice round holes in corks and rubber stoppers.
Go to Ebay and type in "cork borer." You can also find cork borer sharpeners there sometimes. They still make them, and you can buy new ones. The best deals of course are when they sell an old set that they got from a lab that is closing down, or some similar source.
Electric cork borers are available at times, but they might be from somewhat to pretty darn pricy. Hand operated crank ones, that look a little like a cross between a drill press and and a brace and bit, are available, and I have brought one of those home from work, and they do a good job.
If you decide to go the route of the cork borer, the 3mm size is about 1/8 inch, 6mm is about 1/4 inch, 10mm is about 3/8 inch, 12mm is a little less than 1/2 inch, and a big one like 18 or 20 mm is on the order of 3/4 inch. I warn you though, if you use a 18mm or 20mm, it will take a while to cut each popper if you use the manual type rather than a hand crank model.
Another approach is to used a fired hand gun cartridge with the primer pocket drilled out to take a small bolt so it can be used in a drill press or power drill. If you go that route, you may need to make a small slot in the side of the cartridge case so that you can force the popper body out of the case.
No matter what you use, a lubricant like glycern, from the drug store, will help the borer or any cutter to cut more easily. Glycern is a good choice, and is one of the traditional material used for cork or rubber stopper boring, because it works well as a lubricant, and it is water soluble, so water will wash it off the cork, rubber, or foam.
Another thing about boring with a cork borer, or any other borer for that matter, is bore in a bit until it starts to grab or become more difficult to bore, then back out a bit and bore back in and out a time or two to work more of the lube into the slot you are cutting. Sometimes you have to use a plastic dropper to add a drop of lube as you go.
Regards,
Gandolf
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