According to shotgun instructor Gil Ash (www.ospschool.com), here's how you can tell if your gun needs adjustment.
• Mount the gun and have a buddy measure the distance between your nose and the thumb knuckle on your back hand (trigger hand). It should be about one and a half to two and a half finger-widths (your fingers, not your friends').
• When your gun is mounted, the distance between your grip hand, your elbow and your shoulder should all be equal. This forms an equilateral triangle and distributes recoil in the right spots evenly. If the gun's too long, you take more recoil in the shoulder than you should. If it’s too short, you take the extra recoil in your wrist.
• Mount the gun with slight cheek pressure (don't push your face into it). Just underneath the cheekbone where your teeth touch is your anchor point. If your eye perfectly aligns with the rib, the gun fits. If your eye looks down on the rib, the comb height needs to be lowered. It is far more common for a comb height to be too high than it is to be too low.
Most importantly, though, is to practice your gun mount regularly. You don't have to hit the range every weekend to do this — check out the OSP Flashlight Drill HERE. As Gil says, "Gun fit is worthless unless you can mount the gun consistently."