Can I use them in my gun, or must I buy cartridges in 2.3/4in cases, which are more expensive?
You can use the 2.1/2in (65mm) cartridges in your gun with every confidence, and you don’t have to use 2.3/4in (70mm) cases.
In fact, most clay shooting guns these days are built with 3in (76mm) cases, and 3in cartridges have no use in clay shooting.
The guns are just made that way for owners who wish to use them for occasional wildfowling trips, or anything else requiring a very powerful cartridge.
The golden rule is never to use a cartridge longer than the gun’s chamber – for instance a 3 in cartridge in a 2.3/4in chambered gun – or potentially very dangerous high pressures will be generated.
The one exception is that some cartridges loaded in 67.5mm cases can be used in game guns with 2.1/2 (65mm) cases, and the rule in this case is to go by what it says on the cartridge packet.
Finally, remember that the length of a cartridge is the measurement of a fired case, and that an unfired cartridge is approximately 11mm shorter.