You might want to read up on bedding. The best bedding compounds are epoxy resins filled with metal or other substances such that event the small change in shape that epoxy experiences as it sets is mitigated (the filler means the only shrinkage will be in the resin between the particles and not across the breadth of the material). Probably the only commonly available product that comes close to fitting this criterion is standard slow-setting version of JB Weld (don't use the quick-setting type; it is too weak) or the fiber and metal-filled PC-7 that Ace and some other hardwares carry. A properly formulated bedding material, like Brownells Steel Bed or Bisonite should be tougher than JB Weld and PC-7, but JB can work surprisingly well, at least for a time.
You need a good release agent. It needs to be thin, so it doesn't interfere with the bedding getting up against the gun, but still makes sure the epoxy doesn't stick. Cooking spray, like PAM can work. Many industrial suppliers have commercial formulations. I've had great results using Johnson Paste Wax in the yellow can. Wipe it on thick and let it dry a bit, then buff the excess off.
If you have room in the wood, you can run a Forstner bit into the screw hole from the inside to make a widened recess that will accept a stack of washers. The washers can be built up until they just support the barrel where you want it. This poor-mans pillar can then be degreased and epoxied into position in the wood. The JB Weld can be used for the gluing, though the filler will add a little to the washer stack height, so you may prefer to use a clear slow-set epoxy for the washers, like T-88, which is a slow setting epoxy sold by the Woodcraft chain. Boat supply places, like West Marine also have good epoxies available. The clear stuff will hang onto the wood better. Keep the top washer wiped off and put the JB there. Wax up the screw as well as the barrel and screw it together for about six hours. Remove the screw and pop the barrel back out and look for any excess bedding or cement in spots you want to trim off with an Exacto knife. The Epoxy will still be possible to cut at that point. A week later it will be fully hard.
So apart from grinding my teeth, zip tying it down until I can bed the barrel, does anyone have any ideas for a fix? Beyond that, any tips on bedding it so that it still seats nicely with only the one bolt?