Noway has the right idea, but his percentages are off. Full choke is 72%, meaning 72% of the BBs will be in the 30" circle and th4 40yd distance. Modified is 55-60% as I recall. This is for lead shot though..... Remember that steel has a tighter pattern than lead does. Also, the faster and the courser the shot-the tighter the steel shoot is. An example is this, my IC in my SX2 shoots a 75% pattern whith Fiocchi 3" -3 shot, but shots a 88% with the Federal 3.5" High Velocity BBB with the same choke.
The best thing to do is try different loads/chokes and see what patterns best for the gun.
I pulled this out of my files, this is how you do it:
Patterning a shotgun
I use Plastic tablecloth that can be bought in 100' rolls about 40” wide. I use 2 - 2x4's, 6’ long with pointed ends and pound them in the ground almost 40” apart until they are very stable. I staple the plastic on to each side with about 8 staples onto the upright 2x4's. Make sure the plastic is taught across. I add a bold + at dead center for aiming purposes, and then I use a 15" length of line attached to the magic marker and draw the 30" dia circle. I write on the corner the date, weather conditions, wind speed and direction in relation to target, manufacturer of the load, which load(shot size, amount of shot and FPS), and the choke and which gun. I fire from a bunch rest. A hood of a truck with several boat cushions or such will work too. Make sure you have a SAFE area behind the target. If the plastic tears off from the posts because of the wind, generally it is too windy to get accurate results, so wait for a calmer wind day.
I use 3 shots minimum to calculate percentages and groupings. I circle with a magic marker and count each shot hit in the 30” circle and write it on the target. You can either use ballistic tables which tell you the number of pellets per ounce of shot or you can open up a shell and count the total number of pellets in it. Take the total hits and divide by the total pellets in the shell and this is the percentage of hits that hit within the 30” circle. Use these 3 shots for each choke and Each different load. The idea is to look for nice even patterns with the density you are looking for. This may entail buying many manufacturers and different loads to determine the best loads for your gun. To keep it more cost effective, I'd recommend buying only High Velocity loads (1450fps and over) if hunting for waterfowl. But be aware that HV shells have a more shattered pattern (not as evenly distributed/ a broken up pattern if you will) compared to standard shells. But the increased foot lbs. of energy is really worth it especially on big ducks and geese when compared to standard (1400fps and less) loads. One of the things I noticed is how few pellets there really are in the bigger shot loads---it is hard to get a nice filled pattern, but with the HV shells I have gone to smaller shot sizes the last few years than say 3 or especially 10 years ago. I now like to shoot the corresponding equivalent lead shot sizes from 20 years in these new HV steel shot loads. My problem is finding the smaller shot sizes from suppliers in factory HV loads. Also save the plastic targets for future references.
Here is a table of 575fps velocity threshold and the corresponding range at which it drops below the 575 speed. The 575 speed is the point at which the pellet can not effectively kill a bird if it drops below this speed. But this is only part of the picture, as there is the pattern density, EG what is the foot lb. of energy required to kill a bird. The bigger the bird the more foot lbs of energy required to kill it cleanly. The Teal is much lower than a Mallard, or a Lesser Canada and the big Giant Canada goose. And to increase foot lbs. of energy you need 3 things, 1 is faster pellets , 2 is closer range, and the third is the number of pellets that actually hit the bird (pattern density). There is some division on what is required to kill a bird. 1 camp says it takes 5 to 6 hits period to kill a bird. While another says there is a corresponding ft lbs. of energy of a pellet x how many actually hit the bird. I choose to stay out of this argument, but just wanted you to know there are differences of opinions out there. Sorry but I do not have the table for required ft lbs. of energy to kill a specific sized bird.
Steel Ballistic chart of HV shot.
Shot size 6 5 4 3 2 1 B BB BBB T
1450 fps 43 47 51 55 58 62 65 70 73 77
1500 fps 44 48 52 56 59 64 68 71 75 79
1550 fps 45 49 53 57 61 65 69 73 77 81
1600 fps 46 50 54 58 62 66 71 75 78 82
1650 fps 47 51 55 59 63 67 72 76 80 84
1700 fps 47 51 56 60 64 68 73 77 81 86
1750 fps 47 51 56 60 64 68 73 77 81 86
1800 fps 47 51 56 60 64 68 73 77 81 86
Something to consider is that most hunters can not effectively shoot past 40 yds. because of personal limitations, not limitations of the loads they are shooting. There are very few guys that can consistently shoot over 60 yds and cleanly kill birds even when using loads that can actually effectively shoot that far. Remember skybusting is shooting at anything past what the load can perform AND/OR shooting past your personal limitations.
Well this is how I do it, and it works very well for me when pattern testing a gun and load.
The Audacity of Bull Crap.
"Typical: Gun-loving, bitter bible-thumping white person" Barack Obama.
Hey I resemble that comment!!! Those are FIGHTING WORDS!!!