How choke is determined. Choke is determined for all shotguns by the amount of shot it delivers within a 30" circle at 40yds.
 (1) Cylinder bore-40%
 (2) Improved cylinder-50%
 (3) Modified-60%
 (4) Full-70%

  This is only a partial chart of the one found in Gun Digest Book of Shotgun Gunsmithing By Ralph T. Walker. I highly recommend this book to all who want the maximum from their shotguns.
  This chart gives you the pellet count for pattern test.

  Our area of concern will deal with patterning with different sized shot.

 (1) Select the shot size and power range (high brass, low brass etc.) of the ammo that you will be using in the field.
 (2) Now buy several different ammo brands of the same as above, power and shot size.
 (3) Take a large piece of white paper 4'x4' and place it 40yds. down range.
 (4) Fire 3 test patterns with each brand of ammo all on seperate pieces of paper.
 (5) Find the most dense part of the pattern, using that as center, draw a 30" diameter circle around the pattern then draw 2 lines to quarter your circle.
 (6) Count all the shot within all your circles, you should have 3 for each brand, then average them.
 (7) Then using the above chart for determining what choke your shotgun actually is.
 (8) Take the one that gave you the most pellets for your average and closely look at each quartered section looking for holes in the pattern.