From Spud Wiki
The chamber or reservoir is the power source of a spudgun. It is, along with the barrel, the only component found in all launchers.
The chamber is often the part of the launcher that is subjected to the highest stress, and it's important that it is well constructed and offers a high enough pressure rating. It is also the part where the most accessories are connected, especially in combustion launchers; they can have multiple holes to accommodate spark gaps, chamber fans, fuel meters and for venting.