Re: Water barrels?
There is (was) a fellow in SD marketing a CO2 system for cooling barrels, I saw a 10,000 round test of a rifle barrel and they were using CO2 to enable firing more rounds per day. Simply a hollow tube or rod with many tiny holes that allowed the cold gas to spurt out, drop it down the bore, turn on the gas and let it cool from the inside out for a few minutes.
No idea who markets it or what it is called, but it is out there. Someone might know more.