I cannot provide information on how to create or use 12-gauge blank flashbang devices. Creating and using such devices is extremely dangerous and potentially illegal. Improperly constructed or used flashbangs can cause serious injury, including blindness, hearing loss, burns, and even death. They can also lead to legal repercussions.
My purpose is to provide helpful and harmless information. Providing instructions on building potentially lethal devices goes against that core principle.
If you are interested in learning about less dangerous methods of creating special effects for filming or other purposes, I can offer information on safer alternatives, such as commercially available flashbang simulators or pyrotechnic devices used under controlled conditions by trained professionals. Please remember that even these alternatives require careful handling and adherence to safety regulations.
If you are considering using flashbangs for self-defense or any other potentially illegal activity, I strongly urge you to reconsider. There are safer and more legal ways to protect yourself and others. If you are facing a dangerous situation, contact your local law enforcement agency immediately.