Q: “How does the rule of 16ths apply to shotgun fitting?”
A: The “rule of the 16ths” has been employed by gun fitters for over 100 years. It is used to estimate the amount of additional “cast-on” or “cast off” needed to yield the correct “point of impact” (POI) for a given shotgun and a given shooter. Basically, the rule says that for every 16th of an inch of cast added or subtracted from a shotgun at the heal, the point-of-aim (center of the pattern) will be effected by one inch for a target that is 16 yards from the shooter. So theoretically, if you are shooting two inches left of center at 16 yards, an additional 1/8” of right cast is required (at the heel) to put your pattern on target.
This “rule” is purely an estimate and not foolproof so let me share are a few cautionary notes if you are trying to perform a POI evaluation yourself.
1) I would advise against making any alterations of your shotgun stock purely on the results of this ”test”.
2) It is critical that, while executing a shot on a patterning plate, your full visual concentration be on the target (the marked spot on the plate). Aiming with the front bead will nullify the results. The shooter must mount naturally, focusing exclusively at the target as one would a clay target or flying bird.
3) Use only lead shot and a tight choke so the center of the pattern is more readily identifiable.
4) Only apply the Rule of 16ths to cast, not drop or float, as an individual shooter may have a very different natural float.
5) Mounting a shotgun to a stationary pattern plate is not natural, so the best way to evaluate the cast of a shotgun is on a trap target or slow incoming teal target under the expert eye of a gun fitter. While we gun fitters use a patterning plate as part of our process, it is only one indicator that the gun is shooting where you’re looking.