Length of pull will vary with the other dimensions of your stock. For instance, if you have a fairly straight stock, with very little drop at the comb( Run a yardstick or broom handle or other straightedge down the barrel and extend it back over the stock. At the middle of the comb- where your face will make its highest contact to the wood- you measure the distance between that mid point, and the straightedge above. If its less than 1 1/2 inch, you may need more, or less LOP( depending on whether you mount the gun to your shoulder's pocket, or out on the arm). That comb height needs to be adjusted for the shooter. People with long necks tend to shoot a stock with more depth at the comb, or with a stock that curves downward more than people with short necks. Same thing applies to people with broad shoulder.A stock fitter has to take into account the real distance between the shooters pupil and his cheekbone below. Long faces require taller height, or deeper depth, at comb.
Then, you need to take into consideration the shape of the chest of the shooter. Most men, and almost all women, tend to have barrel chests, which require more down pitch, than that required for flat chested( skinny ) men, and some flat chested women. Down pitch is measured by placing the gun against a vertical wall, so that the butt is flat on the ground, or floor. The distance from the wall to the front sight is the amount of down pitch in the gun. For barrel chested people, take more wood off the toe of the stock. If your shooter is stocky, and barrel chested, you may need to take as much as 1 inch off that theoretical measurement, and sometimes even more if the shooter is short, but has long arms.Alsways ask the shooter if the present stock they use leaves a "U" shaped mark or bruise on their upper chest muscles when shooting. If so, then they need wood taken off the toe of the stock so that the chest is taking the recoil forces evenly over the entire surface of the buttplate, and not just on the toe.
That is why stock fitters use what is called a " Try stock", to determine these measurements.
Once you have these three measurements, then you have to have the shooter mount the gun to his face, while you look down the bore at the position of his eye over the barrel. By doing this, you can determine whether his face is thick( okay, mine is " fat ") enough to require some Cast off, or cast on( for lefties), and how much. I have 1/4 inch cast on( I'm a leftie), a little more depth at comb, because of my long neck, some down pitch because of my barrel chest, and need a length of pull of about 13 1/2 inches, even though I stand 6'1" tall. Just remember that stock ftting requires patience, and trial and error. The point of it all is that you want the gun to fit you well enough that when you close your eyes, mount the gun to your shoulder and face, and then open those eyes, your dominant eye is looking down the barrel at the sights, and the target, assuming your footwork is done so you are facing the target correctly in the first place. We try to eliminate muscle strains in the fine muscles, so we don't flinch, or pull the shot off because the gun does not truly fit us very well.Once the comb heeight is correct, the down pitch is correct, and the LOP, and cast off ( or cast on) is correct, the head mounts the stock properly, there is no muscle strain, and the shooter does not jerk his head up off the stock causing the muzzle to rise, and shot over, and behind the target, for a miss.
One of the biggest failures of modern shotgunners, BP included, is lifting their heads off the stock in anticipation of the shot, or recoil. But that is not the real reason you unconsciously lift the head for most people. The real reason is the head is leaning forward too much, or to the side too much, straining neck muscles, and they want RELIEF. So, the brain says, " Hey, this gun is about to go off. Why don't you little guys give yourself a rest and just pull back on this idiot's head and lift it off the stock. Then it won't be pulling on you so much and causing you pain."
With shotguns, there are actually several ways to fit the stock, depending on shooting styles. In America, it is common practice to stock the gun so the eye is looking down the barrel or rib and centers the front bead sight. However, You can also learn to shoot with the eye looking over the barrel or rib by a couple of inches, with the rib and front site bead being seen in the lower portion of your peripheral vision. This allows the head to be higher on the comb, or even off the comb entirely, so that the rear signt is the neck or jaw bones, and not the cheek. With the cheek off the stock, and the eye above line of bore, the shooter is less affected by recoil, and can still hit flying targets very well. This is often referred to as the British system of stocking, but I am not yet sure about that pedigree.Certainly it is used all over Europe by its top shooters.
