Outside dimension of the compression chamber.... well sorta. But I doubt you'd see OD of a compression chamber mentioned a lot. I'd imagine the ID, inside dimension, of the comrpession chamber would be much more valuable. If you can picture a syringe like a doctor would give you a shot with.... the black rubber piece, the seal on the end of the piston would be measured by it's outside, OD. Whereas the cyclinder it goes into would be measured by it's ID. You wouldn't want the cyliner/ compression chamber to measure 3/8" and the piston/ seal to measure 1/4". That would give you 1/8" air space (technically 1/16" on all sides around the outside of the seal)... this void would cause the air or fluid to pass by the piston / seal and would not create the pressure necessary to force the fluid out of the syringe or pellet out of the barrel if talking air guns.
