Most avid shotgun shooters understand the basic concept of chokes and how they affect the shot pattern of shells. Different chokes have different shot spreads in five-yard increments. The average shotgunner usually has three to four chokes they commonly use in their “scatter-gun” — Skeet (SK), Improved Cylinder (IC), Modified (M), and Full (F).
As an NRA Shotgun Instructor, you have a duty to be a knowledgeable shotgun shooter. Patterning is more than just choosing the right choke. Most individuals believe that they will get the shotgun pattern they want according to what is marked on the choke. This is not always the case. It is not widely known, but not all chokes designated for a specific shot pattern actually deliver that pattern.
Just as a rifle shoots better with a specific brand and powder load of a cartridge, shotguns can be quite particular as well. The spread or pattern of the shot is dependent upon the relationship between the choke and shotgun, and how they work together. Additionally, just like a rifle, each shotguns pattern differently with specific brands and powder loads of shotshells. If you are an instructor, you need to know how to measure a shotgun’s bore diameter, identify the actual pattern, and the proper way of using a pattern board.
Measuring Your Shotgun’s Bore Diameter
When determining your shotgun’s choke patterns, it is important to remember that the actual diameter of a bore varies from gun to gun. This means that the actual constriction is going to vary between guns with choked barrels and guns with screw in chokes. The only way to determine the exact choke or constriction of the barrel is to measure. The formula for determining the choke of your firearm is: Bore Diameter minus Choke Diameter equals Constriction (BD-CD=C).
To get the measurements, you need to determine the exact constriction. To find this measurement you need a bore gauge, which is inserted through the muzzle of your unloaded shotgun. A measurement is then taken in the barrel, behind the choke, before the constriction starts. The second measurement is taken in the choke at the point of most constriction. The difference between these two measurements will give you the exact constriction of your choke.
For example: My Remington Express 12-gauge shotgun has a bore diameter of .079 inches and a choke diameter of .098. This gives me a constriction of .019 inches or in other words, a Modified (M) choke. Since barrels vary slightly from each other and chokes vary from each other as well, you could get measurements that vary from .001 inches to .003 inches. That means that you may think you are shooting a Modified (M) because of the choke you have in your barrel, but it may be closer to an Improved Modified (IM) constriction.
Identifying a Shotgun’s Pattern
Chokes are constrictions in a shotgun’s barrel that hold the shot pattern together for a specified distance. These constrictions are shown as “Thousandths of an Inch,” which is equal to 1/1,000 of an inch. Most shotgunners believe that if their shotgun’s choke states it is a Cylinder (CYL), Skeet (SK), Improved Cylinder (IC), Modified (M), Improved Modified (IM), or Full (F), that will be the pattern, or “spread” of shot that comes out of their muzzle.
Choke constrictions for the common 12-gauge and 20-gauge are as follows:
12 Gauge – Cylinder (.000), Skeet (.005), Improved Cylinder (.009), Modified (.019), and Full (.035)
20 Gauge – Cylinder (.000), Skeet (.004), Improved Cylinder (.006), Modified (.014), and Full (.025)
Shotguns either have choked barrels, also known as “fixed barrels,” or they have screw-in chokes. Choked barrels will oftentimes be stamped on the barrel(s) “Imp” for Improved Cylinder, “Mod” for Modified, or “Full.” Screw in chokes come in many constrictions with the most common being Skeet, Improved Cylinder, Modified, or Full chokes. The constriction for screw in chokes will be spelled out or abbreviated on the choke, and/or be identified by notches around the rim of the choke or several asterisks. Each manufacturer has their own unique marks on their chokes indicating the constriction. Always double check the owner’s manual to verify.
There are also gauges on the market that you can insert into the choke of your unloaded shotgun when it is screwed into the barrel and get a measurement in thousandth of an inch. By relying on the identifying marks on your choke or by using a gauge, the best you can hope for is a close estimate.
The Proper Way to Pattern a Shotgun
To determine the constriction, or choke, of your barrel you can also pattern your shotgun. To get the exact constriction, this technique must be performed properly. If you do not properly pattern your shotgun, the results will be close, but not exact.
Understanding choke patterns can be just as perplexing as understanding chokes. You must get rid of the mindset that “all” improved cylinder chokes pattern like an improved cylinder choke, “all” modified chokes pattern like a modified choke, and “all” full chokes pattern like a full choke. First, you must decide if you are only interested in a close percentage of hits on your target or desire a more precise number.
To pattern a shotgun, we use a “pattern board.” Most people think that all you need to do is set up a large piece of plywood or sturdy paper 40 yards away, load a shotshell, aim at a point on target, and slap the trigger. Then, draw a 30” circle around the densest group of pellet holes. Many shotgunners think that all you do at this point is count the number of pellets inside the 30” circle or that touch the line and divide that number by all the holes on the target paper, including the ones inside the 30” circle. This number will give you your percentage of hits.
This is a good way to get a quick idea of your shotgun’s pattern, but it is only a close representation of the percentage of hits. For some shooters, close is good enough. But, if you want to be precise and want to know the true percentage of hits, you need to follow a few extra steps.
Step One: Set up your pattern board as described above. Shoot at an aiming point and draw a 30” circle around the densest amount of hits or holes in the paper. Then count the number of holes inside the circle, including the ones touching the line. Write this number down. You then need to perform these tasks two more times so that you have three sets of numbers recorded.
Step Two: Take three of the same shotshells you just shot to pattern, and carefully cut open the crimping. Pour all the shot, or pellets, into a bowl and count them. Take the total number of pellets and divide by three. This will give you the average number of pellets within each shotshell.
Step Three: Take each of the hits or holes that you recorded in step one, and divide that number by the average number of pellets in each shotshell that you recorded in step two. This should give you three percentages of hits.
Step Four: Take the three percentages of hits that you recorded in step three and add them together. Take this number then divide by three. This will give you the average percentage of hits for your shotgun.
The “Guru of Guns,” the late Jack O’Connor, set the standard for acceptable percentages of shotgun patterns in his book, “The Complete Book of Shotguns and Rifles.” In his book, O’Connor states that the acceptable range of Cylinder Choke is 35%-40%, Skeet Choke 35%-40%, Improved Cylinder Choke 45%-50%, Modified Choke 55%-65%, Improved Modified Choke 65%-70%, Full Choke 70%-80%, and Extra Full Choke is more than 80%.
Understanding the complexities of chokes will almost always make you a better shooter. If you have ever wondered why you missed that easy shot, or if you are having trouble improving your shotgun skills, the trouble might be the relationship between your choke and barrel. By determining the exact constriction of your shotgun, you can better determine the lead ahead of the target before pulling the trigger. It can also help you in determining the distances that you are able to make that quick, clean and ethical kill.
