My 28" full choked 42 is .017. It throws a very tight pattern.
I have .410 guns as tight as .030" and all the way to cylinder. There is definitely a difference in the pattern. You're just dealing with a smaller pattern and less shot. My .030 barrel throws the tightest pattern of any .410 I have. IMO, the .410 is very sensitive to barrel irregularities, smoothness and in particular, the forcing cone and choke angles and smoothness. The forcing cone seems to have much effect on the pattern. My theory is that it's because the .410 has such a high percentage of shot in contact with the bore and shot is damaged as it passes thru the forcing cone, more with a standard cone than a long polished one.
A friend had a brand new Perazzi .410 that was choked over .030". It was very tight patterning as well.
