Posted by Jamie on September 10, 2015, 10:32 pm, in reply to "Re: 2 1/2" 12ga Loads"
After taking with them further, they indicated the UK standard on measuring 18" ahead of chamber. I found this information and it seems to coincide with their measurements.
UK Working Standards recommended minimum wall thickness measured 18" from the barrel breech from Double Gun Classics p. 56, Vol. 1, No. 4, 2006: 2 1/2" 12g - .028 2 3/4" 12g - .032 Re-proof recommended minimum - .024