Here is a die I made to fit a 1" barrel from my Pedal Air Gun.
The 1" wood dowel slides closely down the 1" PVC. I cut slots in it at a 7-1/2*deg angle, initially to accommodate some clothes line wire. Later I recut the slots to fit some 3/16" square stock. The depth is set to make the metal bits about .030"* larger than the ID of the tubing. The ends are smoothed to be a sharp, square edge.
Break the inlet end of the tubing to allow the cutters to get started down the tube, and pound the whole thing through the tube using a broomstick as a pusher. I ran this through the tube 3 times, twice with the wires, and once with the keystock. Here's the result:
All in all, not bad. The keystock grooves are the 4 darkest ones. FWIW, I need to drill some holes at the leading edges of the grooves in the tool to give the scraped off plastic some where to go. Details, details. Fitting the 12ga darts (click the link) with some adhesive door insulation to tighten up the fit inside the barrel should give enough grip to properly spin them. In season, the crabapples should also fit closely enough to pick up the spin. Twist rate by the way is 1:24 which according to the web page on the topic, should be plenty for this size of projectile.
As mentioned before, the smooth bore groups about 5 inches at 10 yards. When the snow stops and the sun comes back out, I'll see how much smaller the groups get. For all of you trying for a match grade spud gun, here's how to do it, or at least how to keep you busy for a while.
*All dimensions are approximate. +/- 10% is not out of reason.