Well, about a year ago, I saw a man at the range who was shooting a .243 put a tube in the receiver end of the barrel, and on the other end of the tube was a threaded bracket attachment. He attached a Crossman CO2 cartridge for a BB/pellet gun. Tightened the bracket much like you would on a pellet pistol, while holding the tube in the receiver, which pierced the cartridge. The pressure release blew the CO2 right through the barrel. Used 2 or 3 cartridges, took him about a minute or two from start to finish.
A few minutes after that, he was shooting his next grouping. He repeated this at least twice more while I was there. I can't say if this was really effective, but he seemed satisfied with the results.
I don't know if this was something he crafted on his own, or something that's commercially available. I've never seen it before or since. I don't know if he was experimenting, or this is common practice for him.
Has anybody else seen this? I don't know the physics of rapidly cooling a barrel, if it's safe for the barrel (I would think warping would be a danger), if such a small amount of cold CO2 can even do the job, etc.
I've always been content to wait 15 minutes or so and shoot one of my other guns while I wait.