A common problem encountered with fifty caliber
rifles intended for shooting beyond 1000 yards is not having enough vertical
scope adjustment to hit the target. Usually it's not a problem to
get on at 1000 yards, requiring about 25 minutes of up adjustment.
But beyond 1000 yards, trajectories fall off fast. Most fifty caliber
shooters that I know, and others shooting cartridges like the 30/378 Weatherby
and 338/416 Rigby, zero their rifles at 100 yards and work off that zero
for all other ranges.
Most internally adjustable target scopes have
from 50-60 minutes of total vertical adjustment. With a normal scope
base and ring configuration, a 100 yard zero will put the adjustment in
about its mid range. With a total of 60 minutes of adjustment, that
leaves only about 30 mininutes remaining. So we can see why we will
soon run out of scope beyond 1000 yards. With the newer bullets now
available, with a C1 ballistic coefficient of 1.0 or better, and fired
at reasonable 50 BMG velocities, the bullet will remain supersonic out
to 2500 yards or so. It takes a lot of adjustment to get zeroed out
where the deer and antelope play, but it can be achieved with certain scopes
and the right base.
The answer is to tip the scope down in front
to take advantage of the full adjustment range. Some manufacturers
of fifties provide bases with their rifles or actions that already have
the slope built into them. But what if we have a rifle that doesn't
have one of these bases, or it is wrong for our scope? Well, there
is a simple formula for determining the correct amount of slope.
Let's say that we're using a 36x target scope
and we find that with a 100 yard zero the scope still has 25 minutes of
down adjustment left. This means the scope won't have those same
25 minutes for long shots where it will be needed. It's easy to figure
how much to tip the scope to get that adjustment back. If we decide
to leave 5 minutes of adjustment, then we want to tip the scope an amount
that would move the scope's reticle 20" at 100 yards. Next we need
to know the distance from the front of the forward base to the back of
the rear base. Or in the case of a one piece base, its overall length.
Now to find the answer, we divide the 20" of desired movement by 3600 (the
number of inches in 100 yards) and multiply by the scope base distance
mentioned earlier. If the base measurement was 7", this would look
like (20/3600) x 7 = .039". So the rear base should be elevated that
amount.
I prefer to mill the scope base on an angle,
as opposed to shimming the rear base to achieve the slope. This does
not put any bind into the scope as shimming would do. The base is
placed in the milling machine upside-down, with the front end elevated
the calculated amount. Here the front of the base is raised so when
it is installed on the action the rear will be higher.
If the action has a flat top, the base is simply
milled flat with an end mill. For a round action, it's also easy
to mill the correct radius on the bottom of the base for the particular
action. Using an end mill that is slightly wider than the base, the
milling machine head is tipped at an angle either to the left or the right,
along the machine's X axis. The exact angle depends on both the diameter
of the cutter and the radius needed. There is also a simple formula
for approximating this angle, if we know the diameter of the cutter and
the desired radius to be cut. The formula is SINE A = R/DR,
where SINE is the trigonometric function, A is the angle of the milling
machine head; R is the radius of the end mill; and DR is the desired radius
to be cut. For example, if we use a 1" diameter end mill and the
action is 2" in diameter, the formula would look like this: SINE A = .5/1.
The angle would be 30 degrees. Actually this formula will not always
yield the exact angle, but will get it within a degree or two. Experimenting
with some scrap steel is an easy way to find it once you are close.
I can offer a few combinations that have worked
for me. With a .980" diameter end mill, 31 degrees will mill a radius
for a 1.75" diameter action. With a 1.25" diameter cutter, 35.5 degrees
will work for a 2" diameter receiver. For the same diameter action,
a .725" diameter end mill should be tipped at 17 degrees.
I hope that I haven't made this too confusing.
In practice, all of this is a simple operation. The scope base is
held both tipped and upside down in the milling machine vise so that a
radius matching the diameter of the receiver can be milled onto it.
In order to achieve the correct radius with conventional tooling, the milling
machine head is also tipped whereby a relatively small diameter cutter
can mill a radius much larger than the cutter's actual radius.
It's probably best not to set the scope so
that it's completely out of down adjustment at 100 yards. Later we
might change loads or muzzle brakes or barrels and find that the impact
point has shifted. I like to leave from 5 to 10 minutes of adjustment
in reserve. Using a conventional internal target scope, the amount
of tip will probably be about .030" - .040" depending on the ring spacing.
With a scope like the Leupold MK 4 with its huge 140+ minutes of total
adjustment, the difference will be more like .100" - .125".
Using this simple formula, we can make minimal
work out of slope calculations and get the maximum from our scopes.
These same principles apply to elevating the rear mount on a Unertl scope,
too. Happy clicking!
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