| Choke is very roughly determined by shotgun manufacturers for all standard bore 10 gauge, 12 gauge, 16 gauge and 20 gauge shotguns by their pattern or the amount of shot they deliver within a 30" diameter circle at 40 yards:|
Cylinder bore = 40 percent.
Improved cylinder choke = 50 percent.
Modified choke = 60 percent.
Full choke = 70 percent.
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.
Using A Lockhart Pattern Gauge:
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 ounce load of #6 lead 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.
1 1/8 ounce load of #6 lead shot = 253 pellets, 70% = 175 pellets, 4.037 square inches, (4.037 sq. in. = 2.009 in. x 2.009 in.).
Make A Score Card For Lockhart Pattern Gauge:
Assigning A Percentage Value To Each Piece Of Shot:
Identifying Bore And Choke Defects:
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.
Choke Overall Effectiveness:
These gauges are made to each shot size, load size and choke configuration, the one below is for our full choke 1 1/8 ounce load of number 6 shot, each square area is 4.037 square inches or 2.009 inches x 2.009 inches. 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.
The Final Analysis: