Nailguns are simple 2 part air tight chambers with a valve in them. All working parts are sealed with O-rings.
Below is a link to my Youtube channel on rebuilding various makes of nailers.
Here is a sample of whats available.
Replacing the Driver O-ring in the Bostitch RN46
Rebuilding the Duo-Fast CN350 and CN325
Replacing the Top O-ring in the Paslode 900420 IMCT