I have a 13 bore Purdey with sixteen chambers. It was converted from flintlock to centerfire by them in the 1870s. They couldn't put cut 13 bore chambers into the barrels because, originally being muzzleloader, the walls weren't thick enough where the chamber needed to be cut. So they cut sixteen chambers in. It was intended for Kynoch thin wall brass sixteens, the inside diameter of which was 14 bore.
The Magtechs work great in my 16/13 Purdey.
Here is a link to a thread on the gun:
http://www.doublegunshop.com/forums/ubbthreads.php?ubb=showflat&Number=94560&page=1
