Regarding ChamberMates, read - http://www.seminolegun.com/
IIRC they have been shut down for quite some time now - business dispute or divorce, not sure.
I have their chamber insert sets in 20, 28 and 410 for use in a host 12GA gun. Very useful gauge reducers, and I have used my 20GA set a lot - quite satisfactorily....just read and heed the instructions.
Hope they "come back", as I want their "16 to 28GA" set.