If it is an actual 12g adaptor, you can fuck that right off, with all due respect, that's tinnitus in a tin. I wouldn't play at a site that allows them.
@Rogerborg there were mag flashes available, they were the same decibels as a 9mm blank but were the size of 12g as they had a load of magnesium that caused a really bright flash.
I met the guys that started ohshiboom at a milsim in like 2014, think they worked for bt during the day. Long time since then so they may have sold/aligned etc since.
@Rogerborgalready have done, been on the receiving end in a very small room (like downstairs toilet size) with no ear pro. They produced a more (Airsoft) thermobaric boom than the crack of a mk5. They aren't readily available anymore as the person who was making them stopped.