Re: CO2 cooling systems
I believe that there was a fellow in S. Dakota selling a CO2 barrel cooling system a few years ago. I also know that H-S Precision used one when they did a 10,000 round shooting test a while back. Sounded like it was simply a length of tubing plugged at one end with many side holes to allow the gas out - not sure what material the tubing was. Other end attached to a hose, then to a CO2 tank. That is how I had it explained to me. Fellow said that he gave it a few minutes on regular intervals. Not sure if there were bushings to center it in the bore, probably not necessary.
Sorry I don't have more details, maybe you could get the info on a varmint forum.