I'm with Researcher on this one.Considering that 12 ga. chambered
guns in Britain can start life with bores anywhere from .710 -.745
all that a stepped brass gauge tells you is that there is a hole at the end of the barrel!
I had a friend recently who took his very nice Lancaster Crown grade SLE to a neighbourhood gun shop and they broke out the brass gauge and told him it had no choke and his only option if he wanted some was to send it for choke tubes.Luckily he gave me a call before shipping and because the bores were .740 he had .005 in the right and .015 in the left. We averted a horror show....
