Formula For Stock Adjustment


Hi Mr. Buck,

Have forgotten the formula for determining how much elevation raises the point of impact. Example: Raise the stock of a shotgun (or the rear sight of rifle) ONE INCH (raises the muzzle ONE INCH), raises the point of impact ‘How many inches’, at a range of sixty yards? Disregard ballistics and gravity.

Am using 60 yards because I dimly remember that minutes of angle, degrees, nautical miles are the units used. Application: The comb of my new gun is low. Want to know the exact effect on longer shots.

Cap
Peterborough NH

Dear Cap,

The traditional formula, attributed to Churchill, is that a change in comb height or cast of 1/16″ will move point of impact 1″ at 16 yards. I like using this distance as you can get a good, dense pellet strike pattern at 16 yards so you can easily judge what 1″ is. At 40 or 60 yards, even a full choke pattern is so dispersed that it becomes difficult to measure with any kind of precision. Using standard mathematics, you can interpolate the 16 yard impact out to any distance you want (note: I said “you” can, not “I” can). Factor in shot drop if you are really fussy.

I really don’t use the Churchill method in setting my guns up. Just about all guns are too low and much cast off for me, so I just layer on masking tape until I get the POI where I want it. Then I put a small mark on the tape right were my cheek touches the stock, take that drop/cast measurement and then get the stock bent or reinletted or shimmed to match.

Where the Churchill method comes into it’s own is where you have to lower or cast off a stock, essentially removing wood. Here you can’t experiment by adding tape, so you do have to make more of a commitment if you don’t shoot an auto with shims. Churchill’s method helps you make this prediction without the use of a try-gun.

Best regards,

Bruce Buck
The Technoid writing for Shotgun Report, LLC
(Often in error. Never in doubt.)

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