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Some Thoughts On Sighting Your Round Ball Rifle
The Three Inch Rule

One of the questions that many muzzle loader hunters often ask is;  "where should I sight my round ball to maintain a reasonable point blank range (PBR) for big game hunting out to 100 yards"?  Usually other variables are included with the question such as;  caliber, charge, powder, lube, etc., etc.?

The answer regardless of variables is simple;  Sight to hit dead on at 100 yards and your maximum midrange trajectory will be right around three inches .   That answer will pretty much remain unchanged for the majority of rifles and in calibers from .36 caliber up to .75 caliber.  They pretty much shoot with the same trajectory given the typical velocities these calibers are shot at.  Another option would be to sight for a point of impact (POI) 1.5 inches below point of aim (POA) at 100 yards and, theoretically, your rifle will never shoot outside a three inch circle at any point between the muzzle and 100 yards.  You and your rifle together will shoot outside that circle, but how far out  is for each shooter to determine by shooting under various conditions.

I don't advocate the use of a .36 cal or even .40 cal for big game hunting, but I included those two just to illustrate the universal application of the three inch rule.

The chart illustrating the three inch rule was generated using William Frenchu's Ballistic program version 4.13 and applying the British Round Ball Table of 1929.  The ballistic coefficient (BC) for each ball was derived using Frenchu's formula specifically tailored to the British Round Ball Table.  

The front sight height setting was .70 inch above the centerline of bore.  Sight height varies with different rifles, but those variations don't effect the chart results significantly. 

The velocities applied to each caliber are based on my own experience with .36, .45, .50, .54 and .58 caliber rifles and the experience of other shooters with the remaining calibers.

Click this link to view or download the chart in pdf format.