There are obviously a mirate of different things you can use. Any will work, as long as you can see where your pattern is at. The thing to remember is that all chokes are tested at the 40 yard mark. The exceptions to this are the C and SK1 chokes. These are patterned at 25 yards. This is how they come up with the % of shot in a 30" circle figures. I understand that you may not shoot these distances, and will pattern your chokes at the average distance shot. So take into consideration that your % of shot inside this 30" circle will vary from the factory specs depending on your yardage. Here are the factory specs from the Trulock choke website...
Patterns are normally expressed as a percentage such as 50%, 60%, 70% ect. This is the commonly accepted method comparing pattern density. In a 50% pattern ½ of all the pellets contained in the shell will strike inside of a circle of 30 inches in diameter. To find the percentage of any given load divide the number of hits inside the circle by the total number of pellets contained in the shell. You can obtain the approximate number of pellets any given load will have from a shotgun shell reloading book or you can open a couple of shells and actually count the pellets. All pattern testing is done at a distance of 40 yards with the exception of cylinder and skeet 1 chokes in all gauges and all chokes for the .410 bore which are normally measured at 25 yards.
The purpose for this is to allow you to select a choke that will throw a pattern that is as large as possible without having the pellets so far apart that the target can escape multiple hits. For shooting both game and clay targets you want a pattern that is perfectly even in pellet distribution over the 30-inch circle. Having said that, and after looking at thousands of patterns over a span of 25 years I could count on my fingers the number that I would call perfect and if these were measured close enough they would not have qualified. In this instance “very close” is good enough. Two exceptions to the preceding would be buckshot and turkey patterns. With both of these you are looking for a tight center cluster of pellets.
One thing to remember –The only thing that is consistent about shotguns is that very few things are consistent. Identical guns with the same degree of choke and using the same shell may not pattern the same. The same load between various brands of shells can pattern differently. Patterns will change when changing from hard to soft shot. Patterns can change when anything in the shell changes such as different wads, powders or primers. What I am trying to get across is that when you change anything such as brands, shot size, or components you will need to check the pattern as it could have changed, sometimes by an extreme amount.
Once you find a choke/shell combination that gives you the pattern you want it should remain reasonably consistent as long as nothing is changed. I am satisfied as long as the percentage stays within a 5% plus or minus deviation.
The below chart shows the relationship between the degree of choke, the percentage and constriction based on lead shot. Keep in mind that the percentages are a guide only. What you actually want is a pattern that is dense enough to insure multiple hits on your target at the distance you normally shoot.
Lead Shot Choke Chart
CYLINDER .000 40 at 40 yds- 70 at 25 yds
SKT 1 .005 45 at 40 yds- 75 at 25 yds
IMP CYL .010 50 at 40 yds
Skt 2 [light mod] .015 55 at 40 yds
MODIFIED .020 60 at 40 yds
IMP MOD .025 65 at 40 yds
FULL .030 70 at 40 yds
EXTRA FULL .040 73 at 40 yds
TURKEY .045 + 75+ at 40 yds
Keep in mind that this chart should be used as a starting point only. Select the choke and pattern it. Change chokes or loads as needed to get the pattern you want. If you do not pattern your gun please feel free to accept this chart as being infallible.
How to Pattern
I like to use paper that measures 4 feet square. Paper this size may be hard to find locally.
If it is not available try taping several sheets of butcher paper or newsprint together. Bruce Buck the “technoid” of “Shotgun Report” suggested using red resin flooring paper. This is available at any home supply store and shows the pellet holes perfectly when viewed from the back. He noted it is available only in 36-inch wide rolls but it cuts and tapes easily. If you use small paper and your gun does not shoot to the point of aim, part of your pattern could be off of the target. You would probably confuse this with a bad pattern. If it helps you, mark the target center for use as an aiming point. I prefer to use a shooting bench when patterning. At this point you are not checking the gun for fit and a rest helps to remove some of the variables. From the correct distance shoot at the center of the target. I would do this a minimum of 5 times on different targets with each shell or choke that is being tested. You will need to draw a 30-inch circle around the densest portion of the pattern on each target. If you intend to do much of this get a 30-inch diameter piece of thin Lexan [Plexiglas]. You can easily move this around on the paper to find where to draw the circle. Count the pellets inside of the circle. Divide this number by the total number of shot contained in the shell and you will have your percentage. Take the percentage from each target, add them and divide by the total number of shots fired for each shell or choke. This will give you the overall average for that test. Sounds like a royal pain doesn’t it? It is, but there are no short cuts if you really want to know what your shotgun is doing.
After you have finished look carefully at each pattern for holes that are big enough to let your intended target slip through. You want a dense enough pattern to ensure multiple hits.