From Spud Wiki
Blowgun Modification refers to the modification of a blowgun to increase flow.
There are 4 possible procedures; often, all 4 are carried out. Listed in order of difficulty.
The nozzle is the small piece on the front of the blowgun, which serves to focus the blast of air, frequently made of brass, steel, or rubber. It also happens to reduce flow. Most of the time, it can be removed by simply unscrewing it.
Removing slack in the mechanism
Most blowguns use a lever to push down on the piston, and typically, this lever cannot depress the piston enough to allow full flow. Easily remedied by sticking a spacer between the top of the piston and the lever. You can insert a Popsicle stick behind the lever.
Enlarging the holes
The inlet hole to the piston slide tube is typically smaller than it can be, and this (clearly) limits flow. The solution is to grind or drill this hole to a larger size. The piston should be removed first, to prevent damage to it. (see diagram)
Reducing the diameter of the piston
The standard piston's diameter is too big to allow full flow around the piston. Remedied by removing the piston, and reducing its diameter where the step exists currently. This can be done using a small file or a dremel tool. Be careful not to damage the O-ring. (see diagram)