The PSI pressure output of a CO2 tank varies with temperture. Each time you fire a paintball, the tank gets a little cooler. You get condenstion on the outside of the tank. You blast away and you can get frost on the outside.
You use it for three shots in a Nerf cannon, the liquid CO2 cools so much it turns literally into dry ice. Frozen CO2.
That cooling lowers the pressure that is being generated to propel a paintball. See the table in the chart below.
With unregulated or straight CO2 fills, your velocity drops a bit with each successive shot. That results in your ball velocity dropping and your point of aim getting higher and higher in elevation as you fire. At some point, you do not have sufficient velocity to get a break on an opposing player and your done. We call that 'shoot down'.
The solution is to regulate the output pressure of a CO2 tank. An expansion chamber has no practical effect. It can't warm up your CO2 tank. A 60-80 dollar investment that will work in a high pressure marker at 600 psi. You start around 1000 psi at room temperature. You get into a firefight your tank output operating pressure can drop below that 600 psi mark. At that point the regulator is wide open and waiting on a suppy pressure above 600 psi. Your muzzle velocity will continue to drop until you can't play.
So in your QUEST for Improved Accuracy, your first upgrade is to compressed air or HPA. Instead of 1000 to 600 psi working pressures with regulated CO2, you have 3,000/4,500 psi to 850 psi. They have a built in pressure regulator. Every shot you fire is at the same pressure and thus velocity. You point of aim is constant. You ability to break a ball on an opponent is constant.
Yes, they cost more because of the cost of the reg and manufacturing requirements to hold 3000+ psi. Still, by the time you spend 60-80 bucks on a regulator for CO2, you could buy a USED 68ci x 3000 HPA tank with two years left on the hydro certification for the SAME MONEY.
Many paintball fields just SELL their HPA rentals after a year. They have no maintainence issues. They sell the used one for what they can buy new ones in bulk wholesale for.