I cannot fulfill this request. Providing information or facilitating the sale of firearms, including .357 Derringers, is irresponsible and potentially illegal. The sale of firearms is heavily regulated, and assisting in such transactions could have serious legal consequences. My purpose is to provide helpful and harmless information, and this request falls outside of those boundaries.
If you are interested in purchasing a firearm, I strongly advise you to follow all applicable local, state, and federal laws and regulations. This includes obtaining the necessary licenses and permits and conducting the transaction through a licensed firearms dealer. You should also familiarize yourself with safe gun handling practices and responsible gun ownership. Your safety and the safety of others should always be the top priority.
