I'm posting this on request of a fellow FM'er. It's a fairly technical article and provides a very rigorous approach to pellet counts and pattern spread/density.
Determining choke by patterning:
Choke is determined for all standard bore 10 ga., 12 ga., 16 ga. and 20 ga. shotguns by their pattern or the amount of shot they deliver within a 30" diameter circle at 40yds.
(C) Improved cylinder-50%
While this tells what choke you have, the really big question of, how efficient is the choke and is there a problem inside of the barrel.
Matching shell to chamber:
With so many universal chambers today (12 gauge = 2 3/4 - 3 1/2 in.), trying to establish a certain criteria by which to test a barrel for efficiency can be very confusing. A good rule of thumb is, pattern test your barrel for its largest shell chambering.
For the all around shot gunner using a 2 3/4 in. chamber 12 gauge:
The are many offerings in shot shells in shot sizes and weights. The most common shot sizes run from #4 - #8 and weights from 1 oz. - 1 1/4 oz. having said and established that, to proof your chokes use a middle of your selections load, [1 1/8 oz. load of #6 shot].
The nature of most shotguns and shot shell combinations would be:
A. #8 shot will pattern tighter than #6 and #4 not as tight as #6.
B. 1 oz. loads tighter than 1 1/8 and 1 1/4 not as tight as 1 1/8.
Even if your shotgun patterns the opposite of the above, the 1 1/8oz. load of #6 shot is still the middle load combination.
Using a Lockhart pattern gauge:
Our first gauge seen below can be made from a piece of Plexiglass that is 30 x 30 in. square. This will give you your choke rating and to help identify problems within the barrel and choke.
(1) Determining the choke:
In a 30 inch diameter circle there are 706.50 square inches of area that needs filled evenly with a small amount of shot, by breaking this area down into 2 even areas we find that inside that 30" diameter circle is another circle that is 21.21" in diameter both having equal area at 353.25 square inches each.
This means if you were using a 1 1/8 oz. load of #6 shot which has approximately 253 pellets or pieces of shot, to shoot a true 100% full choke pattern it would deliver 175 pellets spaced out at one piece of shot every 4.037 square inches.
(A) Full Choke:
1 1/8 oz. load of #6 shot = 253 pellets / 70% = 175 pellets / 4.037 sq. in.
(4.037 sq. in. = 2.009 in. x 2.009 in.)
(B) Modified Choke:
1 1/8 oz. load of #6 shot = 253 pellets / 60% = 150 pellets / 4.710 sq. in.
(4.710 sq. in. = 2.170 in. x 2.170 in.)
(C) Improved Cylinder Choke:
1 1/8 oz. load of #6 shot = 253 pellets / 50% = 125 pellets / 5.652 sq. in.
(5.652 sq. in. = 2.377 in. x 2.377 in.)
Making a score card:
1-A + 1-C = ?
2-A + 2-C = ?
3-A + 3-C = ?
4-A + 4-C = ?
1-B + 1-D = ?
2-B + 2-D = ?
3-B + 3-D = ?
4-B + 4-D = ?
? + ? = Choke
Column 1 represents the core of the pattern its area is 353.25 square inches.
Column 2 represents the outer ring of our 30 inch diameter circle its area is 353.25 square inches.
Column 1 (100) + Column 2 (75)= total number of shot within the 30 inch diameter circle hence in our example 175 total = Full Choke
Assigning a percentage value to each piece of shot:
175 pieces of shot = 100 % of the pattern so each piece of shot is assigned a value of .57 %.
Column 1 = 100 pieces of shot x .57 = 57%.
Column 2 = 75 pieces of shot x .57 = 43%
In this example our full choke has a 57/43 rating. The 57 is the core of the pattern and the 43 is the outer ring from the core within our 30 inch circle.
(2) Identifying bore and choke defects:
1-A + 1-C = ? ? = 1-B + 1-D
2-A + 2-C = ? ? = 2-B + 2-D
3-A + 3-C = ? ? = 3-B + 3-D
4-A + 4-C = ? ? = 4-B + 4-D
How: If you look at the pattern gauge you will notice that each pie shaped piece is numbered and lettered to reference opposing sections of the shotgun barrel. The reason for this is for each defect in the barrel, it will affect that same area on the pattern and the opposing side of the pattern.
Example: We'll say that the numbers are fairly even in sector 2, 3 and 4 but in sector 1-A and 1-C the shot count is a lot higher and in sector 1-B and 1-D the shot count is a lot lower. The reason why this happens is because somewhere in the barrel or choke there is a defect that disrupted the shot from one side to the other such as a dent or rust cavity and even new barrels will show where something has been attached to the barrel using heat and causing ripples in the barrel such as ribs and barrel lugs.
(3) Choke overall effectiveness:
The below gauge is made to indicate the overall performance of the choke remember we are working with a 30 inch diameter circle or pattern of which the area is 706.50 square inches.
These gauges are made to each chamber size and choke configuration, the one below is for our full choke 2 3/4 inch chamber, each square area is 4.037 square inches or 2.009 x 2.009. Once again we are dealing with 175 pieces of shot so our percentage value is .57% for each square area, by simply laying this gauge over the pattern paper you count all the square areas that have no shot in them and multiply by .57% subtract that number from 100% and this gives you your overall choke effectiveness.
We'll say that there are 40 empty spaces, multiply 40 x .57 = 22.8 %, subtract 22.8 from 100 = 77.2 % effective.
The final analysis:
Our final analysis for this example is, we have a full choke pattern that is 77.2 % effective and the pattern configuration is 57 % inner half x 43 % outer half of the total square area of a 30 inch diameter pattern.