Actually a gauge down from 12 is 16, but of course I am being a jerk, since 16s aren’t made any more and I would bet the ammo is hard to come by.
The BPS is available in 16 and is a very nice gun. I have several 16’s and think highly of the gauge. From a 1914 Browning to a custom Connecticut Shotgun RBL.. they are magnificent guns.
It’s the best gauge for upland birds, and nice for close waterfowl like ducks over decoys.
I suggest you check out www.16ga.com.
There’s also a wealth of reloading info for the 16.