Dear Technoid,

I once read about a simple rule of thumb how to calculate the effect of changes in comb height to changes in point of impact: 1/16 inch on the comb would make 1 inch on 16 yards. Do you have any idea on which formula this calculation is based? What would the change be on 32 yards? Surely, such a formula needs to take barrel length and length of comb into calculation (i.e. distance from the eye to target)? Can you enlighten me on this subject?

Best wishes

Anders

Dear Anders,

I first heard this formula from Paul Smith, the English instructor and fitter at Griffin and Howe. If it is 1″ at 16 yards, it would be 2″ at 32. The formula is based on simple triangulation (simple for you, not simple for me- I’m a verbal type and rely on people far smarter than me to do the numbers crunching).

To me 1/16″ at the comb seems like a whole lot, but when you think of it, it would be around 2.5″ at 40 yards. Since a 24″ killing pattern at 40 yards would be exceptionally large, 2.5″ is a pretty high percentage shift.

They use the distance at 16 yards, rather than at 40, because you get a much more clear cut pellet strike at 16 than at 40. This makes accurate measurement far easier. If you have screw chokes, always do your point of impact testing with Full choke for this reason.

The distance from th eye to the front bead should be an important number to calculate the shift, but frankly, shotgun isn’t all that precise. My guess is that you are in the ballpark for anything with a standing breech and 28″ or 30″ bbls. If the Brits really did originate this formula they probably just assume that everyone uses a 30″ SxS English Best.

Best regards,

Bruce Buck

Shotgun Report’s Technoid

(Often in error, never in doubt.)

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The numbers are fairly close in the formula. Certainly good enough for estimating. Using an on-line triangle calculator such as this: http://www.carbidedepot.com/formulas-trigright.asp

gives the following results:

Assuming a 38″ line of sight to front bead (30″ BARREL + 8″ TO EYE ON SxS or O/U – autos=more)

1/6″ =.0625″ calculates the angle A at approximately .094236394 degrees. The angle A represents the angle of the line of sight at 1/6″ above the bore axis.

Using the same A angle, and change the distance to 16yards (576″) yields an elevation increase of .947368″ (pretty close to one inch)

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