If that's the guy on eBay , I contacted him some time back and we went into discussion over the possibilities , with focus on the chamber length on a 3.5 12 gauge to 20 gauge adapter , and a 10 to 12- I have my reservations about someone using a 3" 12 ga adapter in a 12 ga /3.5" chamber firearm , and putting a high PSI/BAR sub gauge load like a 3" 20 ga through it
other than a few back and forth conversations , I haven't heard back from him , and I am not sure on what is being made or offered today
I would say that a shell adapter would be a nice item to have stowed away on the stock of a survival gun , you never know how desperate or how bad your ammo supply could get in a crisis