I don't have a Spanish 12 or 20 but recently bought an Ugartechea 28 gauge. 28 gauge bores are supposed to be .550 inch and the bore gauge measured mine at .550 from the end of the forcing cone to the beginning of the chokes - dead nuts on the standard.
The choke markings are a different story. The right barrel is marked IC and it has .005 constriction - right at the open limit for IC (.004 is SK). The left barrel is marked M and it has .020 constriction - right at the tight limit of IM (.021 is F). I'll decide later, after I've shot the gun a good bit, if I'm going to have the right choke opened up.
I don't consider bore diameter to have any real relevance. Some of my guns are at minimum diameters and some have been reamed and have oversize bores. What is relevant is the bore to choke constriction relationship because it is the best indicator of what the actual choke is (actually shooting and counting pellets determines the actual choke but that's kinda hard to do in the store).
