DCloco mentioned using silicone to plug the muzzle end---DO NOT do this! The process creates hydrogen gas (small amount) and that gas will develop pressure and will pop out your plug from the breech, or muzzle, end if you were successful at sealing the barrel tight. If the breech plug pops-you'll have a mess in the action/trigger area.

*Leave the muzzle end open so that it can vent the gasses.
*Only use 6 volts or less.
*Use a piece of hose attached to the muzzle end so that you can fill the fluid up to a point a little higher than the actual end of the barrel as some of the fluid will evaporate and you won't clean all of the barrel unless it is all submersed in the solution.
*Wrap a paper towel, or terry towel, around the barrel just below this hose & secure it with rubber bands, or painters tape, in case any fluid escapes. These towels will stop any fluid from getting under the stock, and into the action.
*I use a 50/50 mix of straight ammonia & water
*Use a steel rod-the SS may work, but steel will work better.
*Instead of O-rings, or shrink tubing, simply take a piece of 1/2" painters tape and rap it around the rod a couple times spaced out about 8" or so. Then you can cut the tape off easily with a razor blade when you want to clean the rod. Much easier than hassling with the o-rings.
*Use a foam ear plug pushed into the chamber with a fired piece of brass to seal the chamber area

I can't say for certain that it won't harm a barrel if done incorrectly, but I have yet to experience any problems cleaning in this manner. This is the ONLY way I clean guns anymore. After the electronic cleaning routine, it takes one patch with Hoppes #9 to get any left over gunk out, then one dry patch, then one lightly oiled patch for protection. No need for brushes. No need for scrubbing with a cleaning rod.