Maltzie provides excellent advice. The hunting community is slowly becoming aware that how well a fun fits, has a lot to do with how well the shooter will ever be able to shoot it.
There are five rudimentary stock dimensions, 3 cast dimensions, drops at th comb and heel, length of pull (LOP) and pitch. When all fit or match the shooter's size and shape, the gun mount and body posture (shooting form) that is most likely to result in the greatest shooting success can be used.
As Maltzie suggests, some stock dimensions have a greater effect on shooting success than do others. The ultimate goal is to be able to mount the gun (without thinking) and know where the pattern is going when you point at a target, whether clay or feathered.
This seems much simpler than it actually is. Although shotguns are pointed and not aimed, to shoot at a moving target requires something known as a sight picture. It is made up of three points, the shooter's eye, the front bead or muzzle and the target. (It's the only way you can decide when to pull the trigger.)
It is the position of the eye relative to the barrel or rib that determines where the pattern will go. The eye acts like the rear sight on a rifle. If it is too high, the pattern will go higher than expected. Pull the cheek off the comb during a swing and the pattern will follow.
That is why Maltzie wrote that the position of the comb on the stock is so important. It positions the eye, which determines whether or not the gun will shoot where you look, meaning that the pattern will go where you expect it to go and not over or to one side.
As Malatzie says, guns are designed in hopes of fitting the greatest numbers of buyers. That is why stock dimensions are chosen by manufacturers to fit an average man 5' 10" tall and weighing 160 pounds.
It is the best they can do. Unfortunately, not only are there few shooters with that height and weight, there are a lot of other individual differences, even among shooters who are "average".
Without rewriting my book, Stock Fitting Secrets, there is no easy way to explain how to check gun fit and when necessary, explain how to make it fit.
Suffice it to say, you will shoot your best only when your gun fits you. Trap shooters (who tend toward the obsessive compulsive bent), have, been dealing with getting their guns to fit more than those of other shooting disciplines. Skeet shooters come next, followed by sporting clays shooters. Now, hunters are becoming interested in improving their shooting success and gun fit has a lot to do with it.