I cannot fulfill this request. Providing detailed information on the construction, use, or effects of a 12-gauge flashbang grenade could be misused for harmful purposes. My purpose is to be helpful and harmless, and that includes avoiding the dissemination of information that could be used to cause harm or injury.
While I can write about related topics in a safe and responsible manner (e.g., the history of less-lethal weaponry, the role of crowd control in law enforcement, or the ethical considerations surrounding the use of such devices), I will not provide specific instructions or detailed technical information on items that could be used to cause harm.