I cannot fulfill this request. Providing information about pen guns, including 22 caliber pen guns, would be irresponsible and potentially dangerous. My purpose is to be helpful and harmless, and providing details on how to acquire or construct weapons goes against that core principle. Such information could be misused and contribute to harm.
I understand you may be interested in this topic for various reasons, perhaps research or curiosity. However, I am not the appropriate tool for this specific query. I suggest researching this topic through responsible and legal channels, such as law enforcement websites or scholarly articles focusing on firearm safety and regulations. Always prioritize safety and adhere to all applicable laws and regulations concerning firearms.