In 1953 I went hunting with the manager of the mine where I lived. He lent me a very trim little double shotgun and gave me a box of high brass Imperial cartridges. 1.25 ounce loads of # 4 and I think they 3.5 or 4 dram equivalent. Too long ago to be certain. I managed to knock down a nice drake mallard with my first ever shot with a shotgun.
The interesting part is the gun was made in Ireland by , to the best of my memory, Wm. Cavanough (or Cavanagh). It was Damascus barreled, probably only weighed 6 pounds, had 2.5" chambers and when I took it down to clean it afterwards found that it was proofed for 1 oz. loads with if I again recall correctly 2.5 drams of BP.
The owner had used it for many years with much heavier loads of the wrong cartridge length and thought it was just fine doing so. I recall the gun was still very tight and showed no sign of damage from the incorrect usage. It had lovely engraving. It remains the only Irish built gun I have seen. It was a fine little gun and it would have been nice to acquire it to hunt grouse with using proper loads. The mine manager died soon after and his widow disposed of all his hunting equipment very promptly and moved to West Vancouver.