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
need a 2 minute (2 MOA) sight change, we really mean we need to
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
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,