For SxS driven game guns the choke combinations can run anywhere from CYL & IMP to CYL & FULL, depending on the style and type of shooting. For example, driven pheasants (fast) and partridge (faster) coming at the guns, many prefer a MOD or FULL choke for birds at the greater distance, then IC or CYL for the direct overhead shot as the second barrel, the range being much closer. For driven red grouse (the fastest!), #6s or #7s at increased velocities are required. (As you probably know, the Brits in general are not big on variable choke tube devices; they prefer to alternate types and specialties in ammunition instead - spreaders, collared loads for densest patterns, etc.). I would classify them as a "fixed choke nation" with the occasional exception for sporting clays.
For typical driven setups with birds at 35-45 yards, #7 shot is ideal. For ducks at the usual longer ranges, #5 or #6 depending on conditions (strong wind, rain, etc.). The real key is the quality of the ammo. Best quality fibre wadding and high-antimony hard shot make an incredible difference in performance in the field.
I don't think the Brits paid much attention to "squaring the load" in either the 12 or the 20 gauge. When the rules for international (e.g., 'bunker' or 'Olympic') trap were changed sometime in the late 1980s the only specific prohibition for ammunition was that the shot charge could be no more than 7/8 oz. You could have as much powder as you could get into the case, the hottest primers, and whatever 'superwad' you could conger up, but you could have no more than 7/8 oz. of shot. Some of the factory stuff that showed up at the bigger shoots was unbelievable; velocities approaching steel shot specs and wad columns that took up fully half of the shell. Any of them would turn a clay pigeon into confetti at 65 yards from a good second barrel kill. The Italians had the hottest and the largest variety.
Regarding Browning guns and backboring, I don't know anything about modern ATA-style trapshooting and the bizarre modern configurations used for it, but I know that Browning FN true mid-bore diameters in the Superosed and B-25 generation of O/Us measured .723 diameter as opposed to the 'standard' US .729. So with the combination of a slightly smaller bore diameter, superior wadding, hardened shot and optimum proprietary burn-rate powders, you wound up with an unbeatable combination of ballistic efficiency due to optimum gas seal, velocity and patterns.
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