Shooting the same 1 oz #8 1200 fps factory load out of two 28" barrel guns, one a 20ga and the other a 12... What constriction would I have to use in the 20ga 391 to get the same diameter of pattern at 40 yards as the 12ga 391 choked IM? Thanks! John
Get a choke marked IM. And barrel length makes no difference.<br>
The caveat is that not all individual barrels behave the same, but this would be a general rule and the best place to start.<br>
The 20ga strings shot more, but if the chokes are made right, they take this into account by being made for pattern.<br>
If you want actual amount of constriction, you'll have to consult what constriction is being used for 20ga IM.
In the "old" days , the amount of choke ; IM , Mod , IC etc ; were defined by the % of shot that would be in a particular circle at a particular distance . Truth be known , the amount of constriction that provided a particular performance may have been different from gauge to gauge . Another train of thought was that it was more related to "area" than to "diameter" at the muzzle .
Regardless , the only real way to determine how a particular choke tube/barrel or fixed barrel performs is to put it on paper . It is amazing as to how some combination is reported by the shooter who has never been near a patterning plate/frame , much less counted the hundreds of holes involved .