The Brit game gun in 12 - 2 1/2" is another way of doing 16/20 gauge, in terms of performance. American 12 gauges tended to 7 1/2# to 8# guns shooting 1 1/4 oz loads in 2 3/4" cases, just like the Brit fowlers and pigeon guns. For sure you can use such a gun for upland "field" shooting, but it isn't a dream to carry and swing. But, if that is what you have, then that is what you use. 1 1/8 oz in a 16 gauge or 1 oz in a 20 gauge is passable duck medicine, if that is what you have.

The Brits tended to use case length (2 1/2", 2 3/4", and 3") as the separator of shell performance whereas the Americans used gauge (10, 12, 16, and 20).