I agree with Imperdix - there are plenty of 2 1/2" cartridges here. In fact the standard training cartridges for clays are ALL 2.5" (or65mm) length, such as the excellent Hull CompeX loads in 21 or 28 gram loads (that's 3/4 or one of your old Imperial ounces.

I have just come back from shooting high pheasant in the Scottish Borders with my Webley 600 and Hull High Pheasant cartridges, using 30 grams of size 5 lead shot. Works fine. Anything else heavier is unpleasant.

There are four British ammo companies active here: Hull, Lyalvale (also based in Hull), Eley Hawk (now under Spanish ownership and Gamebore. All offer plenty of choices. There are now even a few 2.5" cartridges with steel shot, which is a bit of a challenge because of the need for a bioderadeable plastic wad to contain the 30% more bulky steel shot. Hull have a new one ounce load just released. I have used the Lyalvale Earth Wad loading (roll crimp over a wad) with some success, and have some Gamebore Regal ones to try.

So all is not lost!

HB