My experience is that my 20-bore patterns more tightly than my 12, choke for choke, and I wonder if this is general.
I have read that, choke for choke, all bores pattern the same, but I have to say it does not match my experience.
I wondered why I couldn’t initially hit much with the only 20-bore I ever owned, which was choked 1/4 and 1/2, same as my 12-bore.
The pattern plate soon told me that it was throwing pattern diameters consistent with a 12-bore choked 1/2 and 3/4, although the bore micrometer showed me that the chokes stamped on the barrels were spot on, with 10 and 20 thou of constriction.
I asked a few other 20-bore users how they found their guns patterned, and got mixed results, which suggests that the only way to check any gun’s pattern is to use a pattern plate or – particularly if you are using steel shot – a sheet of cardboard.
The answer, with my gun, was to have 5 thou taken out of each choke.
I wouldn’t take too much choke out of a 20, as with fewer pellets in the lighter loads the pattern could get a bit sparse.
This is the reason why guns of bores smaller than 20 are almost always choked tight, and most .410s have full choke.