The Walther P22′s firing pin is a flat piece of metal that moves freely in the center of the breach block. The firing pin has a nub that keeps it from falling out of the breach block, and it’s held in position by a tiny spring. Only small portion of the firing pin protrudes from the back of the breach block and through the safety block. When the safety is disengaged, there is about a tenth of an inch of the firing pin protruding.
When a the trigger is pulled and the hammer falls, it strikes the little bit of the firing pin protruding through the safety block and sends it forward to impact the chambered round. After the firing pin impacts a chambered, it’s returned to position by a little spring that sits inside the breach block.
When a P22 is dry fired, there’s no casing in the chamber to stop the firing pin and the pin continues on into the empty chamber cavity. This causes the small spring that’s tasked with returning the firing pin to become hyper- extended, thereby impeding its ability to fully return the pin. What ends up happening is that the firing pin isn’t moved far enough back past the safety block for the hammer to strike it. Once this occurs, the pistol will no longer function reliably.
A quick and dirty fix for a pistol that performs poorly due to excessive dry firing is to remove the firing pin spring from the breach block and lengthen is slightly by pulling on it from each end. This should help the problem for the short term, but may also fatigue the spring allowing the problem to resurface.
The best thing to do is to not dry fire a Walther P22. If you must dry fire the pistol, only do so with the safety engaged. The P22′s safety functions by rolling a metal block between the hammer and the firing pin. This will keep the hammer from impacting the firing pin and over-extending the spring.