Any size tubing would work. The darts can be blown out of electrical tubing, PVC, carbon fiber, fiberglass, ali, all of which work well. You can also make you own darts with some practice. Getting them to group and fly consistent may prove to be a challenge. Certainly heavier gauge wire could be used. I have used coathanger wire spun on a grinder to create a point in .625cal configuration. Also used skish-kee-bob stakes as shafts as well as 1/8″, 3/16″ diameter wood dowels. The cone base will be the hard part as a light, flimsy, paper or plastic film cone will not do. Something like a electricians wire nut might work. Or go with the prefabed dart deals although the sites i listed have some interesting darts to say the least.
here is some links. The micro tank or a shimmed tank might do the trick. The idea of the short ones in .40 are hard to say as its all about air volume. There might be enough area to displace the additional air letting you get by with just a 40cal. as far as i know you’d be the first to try. http://www.polar-electric.com/Blowgun/Darts/index.html http://www.geocities.com/caliber625/index.html?200621