It's a letter drill size. There's fractional drills then there number drills but the number series only goes up so high. For the larger sizes that fit between the larger fractionals there's letter drills. Drill Size Chart
of all the fractional, metric, number and letter sizes and the true decimal inch sizing.
Check out Carbide Depot Technical Resources for Manufacturing Professionals
for links to a lot of informative tables. About 2/3 down the listings are links to tapping information charts including tapered pipe threads.
Thanks for the offer but if I need one I've got the lathe and other tools here. I thought this was supposed to be the other way around and I'd end up making one for YOU....
I'm quite keen on the Piranha Pistol deal but I'm looking at keeping it as a vertical holder. I like the much easier access to switching since I use gloves when I play and having the changer out in the clear will make for a lot less fumbling with gloves. Also I like the idea of the 12 being held vertically so it acts like a built in anti syphon set up. But of course lots of pistols use horizontal 12's so I'm probably obsessing about nothing at all. Mostly it's for the ease of swapping while wearing gloves. I'm using light and fairly tight fitting mountain bike gloves but they still make doing things that benifit from tactile feedback a bit more clumsy. Like getting the barrel sock in and out of my pocket and getting the swab out when I need it.