TexasMac's Web Site
By Wayne McLerran
Updated 9/28/13

In my book on Browning and Winchester BPCRs, I devoted a chapter to sight
adjustment and settings, and the impact of sight radius changes due to different
barrel lengths.  Formulas were not provided but I made a general comment that
manufacturers design sights with adjustment scales marked off in increments
of a minute-of-angle (MOA) or fractions thereof.  I further explained that to be
accurate MOA adjustments are based on a rifle sight radius of approximately
34” (34.38” to be more exact).  Hence there will be a “built-in” error if the
shooter installs the sights on a rifle with a different sight radius and assumes
the scale graduations represent MOAs.  Although my comments may be
correct when referring to sight changes in MOAs, the subject is somewhat
confusing and another term, “points”, is used by many shooters in an attempt
to eliminate the confusion.  But some continue to refer to MOAs and “points”
interchangeably, resulting in continued confusion.

One shooter explained it this way.  “Although a MOA is equal to 1.047” at
100 yards, most shooters equate one MOA with 1" at 100 yards and 2” at 200
yards and so on, which leads to an increase in error as the shooting distance
increases.  Hence, the term "points" came to mean 0.010", which is the vernier
scale resolution of most quality tang sights.  We are a lot better at estimating
distances on the target in inches rather than MOAs.  So, while we might say
we need a 2 minute (2 MOA) sight change, we really mean we need to move
the sight 2 "points" or 0.020".  We should toss MOAs out of our vocabulary
because none of us are using it properly anyway.”

Recently I ran across a thread on one of the BPCR forums indicating that rear
sights setting are graduated for sight radii of 36” which, as noted above, is
correct based on the definition of a “point”.  The rather simple sight formula
is: the ratio of sight adjustment (SA) to sight radius (SR) is equal to the ratio
of bullet impact change (BIC) to target distance (TD).  In other words, the
algebraic formula is SA ÷ SR = BIC ÷ TD.  So let’s use the formula to verify
the sight radius necessary to make a bullet impact change of 1” at 100 yards
using a sight change of 1 point (0.010”).  Rearranging the formula above to SR
= SA x TD ÷ BIC = (0.010” x 3600”) ÷ 1.0” = 36.0”.  Hence, with a sight
radius of 36”, changing the rear sight 0.010” will move the bullet impact point
exactly 1” at 100 yards.  But what happens when the sights are used on rifles
with different barrel lengths and the sight settings are discussed without
additional details on sight radius?

Shooters need to understand the differences and make the necessary
adjustments when attempting to utilize reported sight setting.  For example,
shooter A has a rifle with a 38” sight radius.  He provides his sight radius,
load data, velocity and reports a difference of 76 points from his rams setting
to 1000 yards.  Shooter B, a silhouette shooter using the same bullet and
getting similar velocities, reads the report.  He has a rifle with a 34” sight
radius and wonders if his rear sight staff has sufficient adjustment range to
work out to 1000 yards.  To make the necessary adjustment keep in mind that
the shorter the sight radius the less amount of sight adjustment is needed to
move the bullet impact point the same amount.  Therefore the difference in
sight settings of 76 points must be multiplied by the correct ratio of the
difference in sight radii.  In this example 76 x (34” ÷ 38”) = 68 points.  Thus
shooter B’s rear sight staff must be tall enough to adjust up at least 68 points
(0.68”) above his ram sight setting; being able to adjust up an additional 80
points (0.80”) would be better and provide some margin.

In the above example “points” were used to clearly and accurately describe
the difference in sight settings along with the rifles sight radius.  If shooter A
had reported a difference of 76 MOA the actual amount might be interpreted
as a change in sight setting of 0.76” or 0.726” or 0.796”, the latter two values
determined by how the correction factor of 1.047/MOA is applied.  The
exclusive use of “points” eliminates any such confusion or ambiguity but the
rifles sight radius is still an important factor and must be understood when
discussing or conveying sight settings.

Wishing you great shooting,