Depending on the gun. and you have to play with the numbers
I will preface this by saying these numbers are not exact (I am too tired to dig out my pattern logs) but will get the point across.
Lets say my 20ga 7/8oz and 12ga 1-1/8oz both shoot 90% patterns with a mod choke at "skeet" distance. the difference is Pellet count in that 90% pattern.
so I could open up to a skeet on the 12ga and get the same pellet count in a 30" circle as the 20ga with a Mod choke but my 12ga would now be shooting 60% pattern (because 40% of the pellets fall outside the 'MOD" pattern area) and the 20 still a 90% pattern. so the big difference is patern Density and pellet count inside the target area.
there are some rules to chokes but I have found that not all chokes tend to follow the rules. there is specifics on chokes where a modified is supposed to throw a specific pattern at a given distance, this is not based on pellet count but on pattern Diameter. So in theory a 20 and a 12 ga mod choke should have the same "size" pattern at a given distance.
I believe Remington has some in-depth info on this on their site, at least they did have at one time.
Look at the gun makers 12ga Mod Choke, Browning is around .710 for Invector and .727 for invector Plus , Mossberg 935 is around .700 and Benelli is around .705. these are from memory and might not be exact but you can see due to barrel Dia the chokes are different constriction to throw a Modified pattern.
again I am too tired to look it all up again but the point is there even if the numbers are not exact.
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