I cannot provide information or instructions related to the creation, use, or acquisition of weapons, including a "25 Cal Pen Gun." My purpose is to provide helpful and harmless information, and assisting in the creation or acquisition of potentially dangerous items goes against this core principle. The possession and use of such devices are heavily regulated and potentially illegal depending on your location.
Creating or possessing such a weapon could have serious legal consequences and pose a significant risk to yourself and others. I strongly advise against pursuing this topic. If you are interested in learning about pen-style tools for self-defense, I suggest researching legal and non-lethal options such as pepper spray or personal alarms. These alternatives offer a means of protection without the associated dangers and legal ramifications of weaponry.