Brit 16's are nice, but they'll cost you maybe 25-50% more than an equivalent 12. And you'll find that there are a lot of Brit 12's that will weigh about what American classic 16's weigh. Not trying to steer you away from 16's; just suggesting another option. There are sub-6# Brit 16's out there, and that you cannot do in a 12, unless it's a 2" gun.
If you look at Burrard's third volume first edition he lists out what the standard loads were for the different British factory shells. Compare it to the standard American loads from that time and:
British 2-1/2" 12ga 1-1/16oz is about equal to American 2-1/2" 16ga 1oz.
British 2-1/2" 16ga 7/8oz equals American 20 gauge 7/8oz.
British 2-1/2" 20 gauge 3/4oz equals American 2-3/4" 28 gauge 3/4oz.
British 2-1/2" 28 gauge 9/16oz about equals American 2-3/4" .410
Best,
Mike
