Ditto on the cirlcle template... Best wheel-masking tool ever made..
Also, what's a good way to hollow out the barrel of Tamiya 50 caliber machine guns. I'm thinking of a pin vise but this seems not all that logical.
Actually, that's about the ONLY way to do it easily, IMHO... The trick is keeping in theCENTER... That's a little tougher.. Sand it flat, first of all.. There's a mold-seam there that'll throw you off every time..
Then, drill a "pilot hole" using a smaller bit (around a 78 or so), then use successively bigger ones until you've reached the diameter you want.. Depth is NOT as important as keeping it centered, as adding a drop of flat-black will give it the needed depth..