GregSy, around here you get either 12g or .410 shotgun ammo.
The .410 is a very popular snake gun to have & use around the homesteads, out buildings & the fowl house. A plus on the .410 is that Mum is confident using it while Dad & the boys are not there. For this reason most of the locally stocked shells are #6 or at best #4.
While the 12g is a standard which most have come to accept as the only choice & unless a dedicated gun nut like us they might not even know about the 28, 20 or 16. The 10 is an outlaw. Any ammo or reloading components other than 12g is definitely a special order accompanied by wait & cost, so 12g it is.
O.M