I recently came across a Hollis & Sheath (1849-1861, Boothroyd) percussion double. The barrels look pretty clean and I measure the bores at .674 & .683, somewhere between a 14 and a 16 bore. 29" barrels, 7lbs.-8oz.
The best I can figure is a wall thickness of .050 about halfway down. I also got short choke cones of .010 &.020 but choke boring wasn't supposed to be introduced until 1874 by Greener.
Were there such animals as 15 bores or were percussion guns made on a different methodology?
thanks,
Rob




