As others have said, sighting at 25 yds gets you on paper and interestingly, a
25 yd sight- in will often get you very close to a centerfire rifle's
optimum hunting trajectory, or Point Blank Range for that caliber, which in consideration of the rifle's power, may be a shorter distance than the rifle's maximum point blank range. (MPBR),
which is a very useful concept for a hunting rifle.
MPBR... that's the distance above or below the line
of sight which the bullet will move during it's time of flight. For
deer sized game and larger, that's usually given as +/- 5"... and the MPBR is
the distance that your rifle will shoot without the trajectory of the
bullet going above or below 5 inches from your line of sight.
1: A standard .30-06 factory 150Gr load @2920 fps will provide a MPBR
of ~335 yds... the rifle is actually sighted in @286 yds, is .3" high
@25 yds and a full 4" high at 100yds. The trajectory of the bullet will
be no more than +/- 5" high or low from zero to 335 yards... visualize a
10" pie plate covering a deer's vitals and aim for the center... that
will put a shot in the deer's vitals out to 335 yds.
What if you want
your rifle to fire along a trajectory which deviates no more than say,
2" above/below the line of sight, i.e. a flatter trajectory, Point Blank Range? Using the same .30-06 factory load:
2: .30-06 150 Gr SP, 2920 fps: 200yd zero gives +/- 1.8" to 230 yds,
.3"low@ 25yds and 1.8"high @100 yds. The new 2"PBR is good to 235yds, while still maintaining a 5"max drop MPBR out to 275 yds.
Example 3: Same load as above with 25 yd. zero
: 2.8"high @ 100yds (max height 3.3"@162 yds,) 4.7"low @300 yds. Not bad.
It's easy to see that you can tailor your rifle's Zero to match your purpose.
friend swears by sighting in all of his centerfire rifles at 3" high@100yds.
While his method may not produce optimum MPBR or the flattest
trajectory for all purposes with all rifles, it just works well for just about any
centerfire hunting rifle, as does a 25 yd zero.
Sighting in at longer ranges allows you to really fine tune your rifle's sight- in, as the greater distance allows you to correct small errors in windage and elevation which might not be apparent at 25 yds.
Target and tactical shooters aren't so concerned with MPBR and dial- in elevation adjustments to match the target range.
AR15 iron sights are 2.6" above bore
axis and trajectory to line of sight relationship changes because of the
high sights. As example, I have a scoped AR with sight height 2.94".. I have a
load which is sighted in 1"low @25yds, which gives me a 300 yd zero and 5" MPBR of 350 yds. Wouldn't think of shooting at game at that distance with an AR, but it's good to go for zombie attacks.
Iron sight .22 rifles sighted in at 25 yds are also zeroed @ 50 yds, with about 6" drop @ 100 yds.
a great link for figuring out different trajectories with your rifle
and ammo and tailoring your sight- in zero for your purpose: http://www.jbmballistics.com/cgi-bin/jbmtraj_simp-5.1.cgi
Edited by Alan Robertson - December/09/2014 at 11:18