III. Firearm Information by Type
     E. Shotguns
        4. Miscellaneous
           a. Shotgun Fit Parameters

by Scot E. Heath (scot@hpfistu.fc.hp.com)

Here is a brief description of the typical shotgun fit parameters:

Typically , a shotgun is adjusted for:

  Length of pull
  Comb drop
  Stock cast
  Pitch
  Cant

All these parameters are dependent upon proper gun mount and proper
stance.  If you have doubts, get some instruction as to proper gun 
mount/stance.

The length of pull, cast, pitch and cant, should remain very consistent gun 
to gun.  The drop is somewhat different, depending upon the rib configuration.
Remember, your eye is your rear sight.  The angle the bore line makes with 
the line of sight from your eye through the bead to the target determines 
the vertical position of the shot string relative to your line of sight.  
If the rib on the gun already has some angle (farther from the bore centerline
to the top of the rib at the back of the rib than the front), you do not need 
to raise your eye above the rib surface as far to get the gun to shoot as high
as one with no angle on the rib (or no rib).  I believe the best way to 
determine drop is to first get the length of pull correct and then pattern 
your gun.  With this information, you can determine the amount of delta drop 
necessary to move the center of the pattern where you want it relative to your 
sight picture.

The correct length of pull is defined to be such that with the gun mounted,
a space of 1/2" to 1" exists between the nose and the thumb on the stock
grip hand.  This is typically checked by mounting the gun and checking to
see that 1 or 2 fingers will fit between the nose and thumb.

The hardest adjustment to make is cast.  This is the amount the stock (or
comb) is left or right of the bore centerline.  To determine if you need cast
adjustment, with the length of pull and drop set correctly, mount the gun
with your eyes closed.  Open your eyes and see if the beads are perfectly
in line with each other in the horizontal plane.  If you need to move your
head slightly left or right, the cast should be altered.  If you need to go
to the right (whether you are right or left handed) the stock needs to be
"cast off".  If you need to go to the left, the stock needs to be "cast on".   

Pitch is the angle the rear of the stock makes with the bore line.  If it
is 90 deg. the gun has zero pitch.  When the gun is resting flat on its
butt, the barrel typically leans either towards the rib or forearm side.
The ideal pitch is such that when you mount the gun, the butt (or recoil
pad) contacts your shoulder evenly in the vertical plane, i.e. it does not
dig in at the heel or toe of the stock.  Pitch can be affected
significantly by recoil pad selection and should be adjusted at the same
time length of pull is adjusted.

Cant is the "twist" of the gun when mounted.  Twisting the gun results in
your eye not being straight above the bore centerline.  The more you "cant"
the gun, the farther off it is and the higher your eye above the bore
centerline, the worse the effect of cant.  To correct for cant, the butt of
the gun is made to be at the shooters preferred angle while the bore and rib
centerlines are perfectly vertical.  Again, the best way to test for cant
is to mount the UNLOADED gun with your eyes closed and have someone view
you from the muzzle of the gun.  If the gun is "twisted", cant adjustment
should be done.

Both cant and cast can be compensated for by the shooter by altering mount
and head placement.

The following can be done to make all the adjustments listed above:

Length of pull:

  Shorten stock by cutting.  
  Lengthen stock with spacers or different pad.
  Install adjustable hardware.

Drop:

  Remove comb material to lower.
  Add layers of material (typically vinyl, cloth, moleskin) to raise comb.
  Install adjustable hardware.

Pitch:

  Cut stock at desired angle.
  Install angled shims.
  Change recoil pad.

Cast:

  Remove stock material.
  Add layers of material (typically vinyl, cloth, moleskin).
  Install adjustable hardware.
  Have stock bent.

Cant:

  Install adjustable hardware.
  Have stock bent.

Each of these methods varies in price, durability and aesthetics.  Your
approach should fit your goals.  Please don't hesitate to contact me if you
have any questions.

-Scot