Thank you to all that took the time and effort to reply. And thank you to a special forum member that scanned and then e-mailed a 3 page article describing this method of impact testing for gun fit written by Michael McIntosh for Gun Dog Magazine.
For those interested in this sort of thing another respondant on another BBS wrote the following:
"Patterning is both, finding the guns POI (point of impact) and then setting the POI to meet your needs. Last evaluate choke/load at target distance.
Just like a rifle, you need to know where the gun/barrel(s) shoot. Most would bench the gun at 13 yards with the tightest choke available. Leaving small patterns related to small aim points and easier to measure. Place 3 to 5 rounds side by side on paper to confirm where the gun is placing shot while looking down the rib using your preference. if you know what POI setting you need and the barrel is centering horizontally, you can move on to patterning at target distance. Having an adjustable comb and or rib makes quick work out of setting the POI to your requirements. Knowing your requirement is much more difficult if you have never set up a gun to your needs or experienced a gun that shoots to where your looking. Again, making this easy if you do have a gun that shoots well for you. The easy part is to 13 yard pattern that gun and set the next one to place shot to the same place.
For example if the gun is set to place shot 2 inches high at 13 yards, you can multiply that 2" by 3 for 39 yards (= 6" high). In gun club talk thats about 70/30%. Patterning at target distance should prove the point and you will be able to see on paper what the pattern looks like in relationship to a fixed point of aim. Some shooters drop 3 or more rounds on the same pattern to get better coverage and evaluation. Many shooters find they pull the shot off to one side, hight or low offhand. Could indicate that you have changed your stock mount from the bench to offhand. If you can confirm a change in the mount during the shot, you will be able to re-set the comb to compensate.
If your at the beginning of a shooting carrer and have no idea what will work for your needs, you need to take the gun to a trap field and shoot straight-away targets to set the comb vertically to center targets. This can be done on station 7 of a skeet field also, but shoot targets and read target breaks. Average hitting the bottoms, raise the POI, tops, lower the POI. The conversion from paper to moving targets has only the difference of your movement the move to the target. Raising targets must have some vertical lead that can be set from the comb, horizontal is your responsibility and reflects timing, both trigger and speed of personal movement .
Evaluating the gun fit is an entirely different aspect of the game. Has little to do with POI setting, but has much to do with shooting well. Your form, both mount and stance is in consideration. Your physical size also part of the the process. You need to order Rollin Oswalds book to learn for yourself or go to a pro-fitter. BUT...it is the difference between shooting average and shooting well."
