Expansion chambers is a way to ensure your gun runs on gas co2 and doesn't freeze.
On many of the original co2 guns there is a chamber which lets the co2 expand before it gets into the valves. What looks like a regulator on original autocockers is just a hollow tube to allow the co2 to expand. phantoms used to use an over sized bottom line or double bottom line to do the same.
Liquid feeding co2 into an expansion chamber is why the old co2 gun were so consistant. Running my phantom pump on air to get 280 fps give chrono ranges between 268 and 284, running it on liquid c02 through an expansion chamber I can get it to 276 to 280.
Feeding liquid co2 also stops your gun from going hot if it does accidentally feed liquid from a syphoned bottle. (and it does happen)
Really showing my age now I guess.