The most common of these are anti siphon tanks and expansion chambers. Anti siphon tanks have tubes in them that are located at the top of the tank. They must be custom fitted to each gun because the ASA fittings are not standardized for this. This minimizes the possibility of liquid co2 getting drawn into the gun. Expansion chambers help to turn liquid co2 into vapor before it reaches the gun.
Some expansion chambers are much better than others.
I use the ACI units which I believe are the best on the market. These two devices do a good job of supplying gas, rather than liquid to the gun. But they do nothing to control the pressure to the gun. These devices let the gun work over a temperature range of about 40 to 85 degrees. Velocity variations can still occur, but the gun will function.