Patterning a shotgun is usually done by aiming and shooting at a centre dot,(POA (point of aim)) drawn in a fairly large piece of paper, usually at 30 yards. After the shot you can draw a 30" circle around the centre dot, (although not really needed) and then a horizontal crossline even with the centre dot. A vertical one can alos be drawn. It looks like a big riflescope crosshair when done.
Then count the number of shot holes above the horizontal line, then count the number below the horizontal line. Calculate the percentage of shot that hit above the line vs below. This will tell you if the gun is shooting high, flat or low. You might hear a fellow say his gun is shooting 70/30, or perhaps 60/40. This means that his gun is patterning 70% of the shot high (above the point of aim), and 30% below POA, or 60% high vs 40% low. A flat shooting gun is often refered to as one that shoots 50/50 patterns.
If you want to, you could count the shot holes on the right side of the vertical line, and compare to the number on the left side, this will tell you if your gun is shooting more to the left or right. (Believe it or not, some shotgun barrels are bent, and in some cases, threaded choke tubes are not centered properly, and can cause missed targets) Other reasons for lopsided patterns, could be gun fit, or improper gun hold.
The 30" circle gives you a pretty good idea if your load testing, as to whether your getting a good even distribution of shot throughout the circle. Some loads will work better than others, as some chokes will. Experimenting with different loads/chokes, is a good way to help decide which is the best combo for your gun, depending on what you are using it for. You can try this a further ranges 40 yds, 50 yds, 60yds, so you can really see what spread, or density your getting from your load/chokes.
Have fun, and don't forget your ear plugs!
