Pounder
Many British naval and army artillery pieces of this period continued to be categorised by their pound rating, the weight in pounds of the shell that they fired, rather than by their bore. A gun firing, for example, a 32-pound round was described as a 32-pounder, abbreviated pdr. Larger guns, such as the RML 9 inch 12 ton gun, were more often categorised by their bore. This system was used until after WWII. A rough pound rating to bore conversion for that time is 1 pounder-37mm, 2 pounder-40mm, 3 pounder-47mm, 6 pounder-57mm, 17 pounder-76.2mm, 25 pounder-87.6mm, 60 pounder-127mm.
Read more about this topic: List Of British Ordnance Terms