From practical point of view this is tied to original mechanical condition of the gun, chamber length and the WEIGHT of the firearm. For 3" gun shot weight was 1&3/8-1&1/2oz with corresponding gun weight of 7&3/4-8&1/2lb. For gun between 7&1/4-7&3/4lb maximum of 1&1/4oz shot charge should be used. The cartridges with C.I.P proof standards should be employed not some US made Turkey or super duper pheasant loads. In USA we play with lighter payload weight and lower velocity numbers when matching US cartridges for our European guns.
In America if one has gun like yours we go to closest places that carries cartridges (usually place called Wallmart where they don't sell wall paneling or wallpaper but sell food and many other every day things) and buy few boxes of Federal Game-Shok, Winchester or Remington 1 oz or 1&1/8oz load at avg. vel of 1250+/-50fps and go hunting. No Turkey, Wildfowl or crazzy Pheasant loads like NitroMags or Praire Storm,....
