OK, this may be quite long, unlike my usual short and concise posts
What follows is classical shotgun shooting, originating from field shooting. I am quite certain that some olympic coach somewhere has a more modern theory, but this one will still work. This is for right handers, so reverse it all if you're left handed!
First the stance and mount. Stand with feet about 12inches apart, then transfer almost all your weight to the left foot. Position your chin directly over your left toes (if it's not there already). Your right foot should still be flat on the ground, if the heel has lifted bring your right foot closer to your left until you can keep it flat. This is the basic body position, and you will shoot "straight" shots along a line projected from your right heel through your left toes. If you try this you will find you can rotate at the hips about the same amount on both sides. Your feet never move.
Next hold the gun vertically, muzzle upwards, in your right hand - fingers off triggers!. Raise your left hand, palm upwards, in line with your eyes and a comfortable distance out. Now raise your right hand to just above nipple height. At this point if you rotate your right wrist forward, the gun should drop to the horizontal, landing in your left hand and being just about perfectly positioned. Your right cheek should be almost touching the stock so a very slight movement will bring it in contact and your eye should be looking straight down the rib. A slight hunch forward with the right shoulder to cradle the stock and you're ready to go. Your upper body NEVER VARIES POSITION. Consider this position, once acquired, to be locked in place so that you are always looking straight down that top rib. Any and all movement required to acquire or track a target comes FROM THE WAIST, which acts as a universal joint and allows the (locked) upper body to rotate and elevate.
The above should explain why gun fit is so important. The motions above should result in a comfortable and stable mount, exactly the same every time, and with the upper body (and feet) locked. Having established this position, practice getting to it from the ready position. Any shuffling or squirming will result in a varying mount and hence varying accuracy.
So, to gun fit. The are five measurements. Stock length, cast off, drop at comb, drop at heel and pitch of butt. Many cannot be changed on a factory gun.
Stock length (length of pull) is measured from the trigger to the middle of the butt. Correct fit is when the thumb/finger around the wrist of the stock are about 3/4 inch away from your nose. Any closer and you risk them hitting your nose under recoil, and any further away is pushing the gun out from the body and therefore reducing your control over it. Stocks can be shortened, or recoil pads added to extend them.
Drop at comb is obvious, and should be such that your eye looks straight along the rib when your cheek is on the comb.
Cast off is how far the butt is offset from the line along the top rib. It allows for the fact that the pocket your shoulder makes to cradle the butt (in the position described above) is to the right side of your eyeline.
Drop at heel is really defined by drop at comb and pitch of butt.
Pitch of butt. This is the angle of the butt of the gun. When the gun is in the position described above, the butt should make firm contact with your shoulder along its entire length. If there is too much pitch, the top of the butt (the toe) will be in contact but the remainder may not. This will tend to result in the stock lifting under recoil, as it pushes up and back over the shoulder, with the cheek getting whacked. Too little pitch means the heel of the stock (which tends to be quite "pointed") concentrates the recoil resulting in pain and/or bruising.
Of the above, stock length is probably the most important, and most easily adjusted. Drop at comb is harder to adjust, unless you use a super modern gun with an adjustable comb, but quite important. Probably the next most important is pitch, where pain and or poor accuracy will result if it is too far wrong. It's even more important in Australia whens it's hot, with minimal clothing worn and so minimal protection from recoil. Which reminds me, if you're being really fussy, check the gun's fit whilst you are wearing what you be shooting in. An extra inch of clothing can change everything!
If anyone tells you the stock length should be equal to the length of your forearm from elbow to whatever, don't believe them!
If you are ever lucky enough to go shooting driven birds with a loader, the above mount will set you in good stead. When you have fired (one or both barrels), simply lift your right forearm to raise the gun to vertical, leaving your left hand palm up and empty in front of you. Your loader will take the fired gun from you with his left hand, and with his right hand he will place the new gun in your left hand. Drop you right arm back to the wrist of the stock, and take your next shots. Oh, you do have a matched pair of Purdeys, don't you???
And remember, you PULL the trigger(s) on a shotgun, not squeeze!.
