Index Home About Blog
From: (Bart Bobbitt)
Subject: Re: Springfield 1903
Organization: Hewlett-Packard Fort Collins Site

Kwong Wong (blkcat!Kwong.Wong@uunet.UU.NET) wrote about the B&L scopes:

: Do you have any specs on the Bausch & Lomb 36X like elevation range?
: I was really looking for a variable power if possible since I want to
: be able to dial in elevation from 100 out to 1000 yards and still leave
: some extra for sighting in with different kinds of ammunition.  A friend
: has a 6-24 B&L scope.  Nice optics, but only about 30 MOA elevation
: range.  He can get out to 1000 yards, but can't bring the scope back
: down to 100 yards.

Variable scopes typically have less adjustment range than fixed-power ones.
That's because of the extra mechanics needed to move the extra lenses.  A
25% less adjustment range for comparable maximum/fixed power is common.

With a 15X or greater power scopes, you have to use scope bases that
have a 20 to 30 MOA down slope in them.  This is the only way the scope
will be near boresight (or zeroed at 50 to 100 yards) and have enough
elevation adjustment to zero at 1000 yards.  Some .308 Win. loads need
50 MOA of elevation to zero at 1000 yards near sea level when the temperature
is in the 50 degree F range.  That means one needs 50 MOA of adjustment;
typically more than a 20X fixed power scope has.  With magnums and other
flatter shooting cartridges, 40 MOA can be enough.  So, you get to decide
what compromises you're willing to make.

But there is a way out; use an externally adjusted scope such as a Unertl.
Their adjustments are the finest made for externally adjusted scopes.  And
there's plenty of it, like about 100 MOA.  The elevation adjustment has
200-in. of movement.  Each click moves the adjustment .0005-in; with a 7.2-
in base spacing, that's one quarter MOA per click.  The only thing about
these scopes (and the Lyman, older Fecker and Litchert ones, too), is when
the front base is on the barrel, accuracy suffers somewhat.  Folks who
used these scopes with the front base extended from the reciever had pretty
good results this way; but the scope was centered quite high above the bore
and the stock's cheekpiece needed to be high to get good face contact and
notched quite a bit for the bolt to come back into.  On highpower rifles,
the recoil spring must either be removed or set forward so it doesn't push
the scope back.  More than a few folks have shot a highpower rifle with
the recoil spring `activated' and had the eyepiece drive itself into their
head about one 14th of a second after firing a shot.  So, you have to pull
the scope back an inch or so after each shot, then twist it in the same
direction so it will be in the exact same place for each shot to be accurate.
The critical thing in mounting one of these puppies is to be sure the
elevation adjustment is about 5 MOA below boresight when it's bottomed.  If
not, then use a different height front or rear base.  Otherwise, you won't
get the benefit of their 100 MOA elevation (and usually windage, too) range.


From: (Bart Bobbitt)
Subject: Re: Hot .308Win Load?
Organization: Hewlett-Packard Fort Collins Site

Jeff Boone ( wrote:

: The first time this year (Memorial Day), the bullets
: were passing through the target sideways (Sierra 155gr. Palma @ ~2850
: fps).  The scores that weekend were low enough that I decided not to
: shoot the 2nd day.  Last time (4th of July), I used the Sierra 180gr MK
: at around 2610fps.  And still they were reported as subsonic, although
: the scores were a LOT better.  The gun is a pseudo-match rifle in .308.

My first two questions are:

   *  Were the loads chronographed?

   *  Or is that muzzle velocity from a manual for the load used?

The reasons behind these questions is rifles can easily have a 200 fps
difference between what a manual says a handload should give and what
they actually shoot the bullet.  A 200 fps difference in muzzle velocity
means near 80 inches of vertical change in bullet impact for most .308
Winchester loads used at 1000 yards below 1000 feet altitude.

Second bunch of questions:

   * If you used metallic sights, did you calculate the actual movement
     on the target per click based on your rifle's exact sight radius?

   * If metallic sights were used, what make of sight did you use?

For these questions, the reasons are two fold.  Most rear sights have a 40
TPI lead screw for their adjustments and have 12 clicks per revolution of
the knobs.  This means that with a 30-inch sight radius, every four clicks
is exactly one MOA on the target.  If the sight radius is 33 inches, then
four clicks on the rear sight moves impact only ~.9 MOA on the target.
This ~10% error can wreck havoc when using ballistic programs and the drop
for a given bullet is used to determine rear sight elevation requirements.
And some rear sights, such as Mo's, do not move the aperture .00833 inches
per 4 clicks like the Warner, Redfield, Swenson and older Lyman ones do.
They move about 10% less; something that has confused coaches when they
tell two shooters on the line to put on 10 MOA left and the one with a
Mo's rear sight hangs a 9 at 3-o'clock and the one with a Redfield drills
a center X.

Some scope sights don't move impact exactly what one thinks they might.
a 5% error (or more in some) can cloud sight setting data when compared to
ballistic software or tables.

Sierra's 155-gr. Palma bullet needs to leave the muzzle at at least 2900 fps
to be sure it'll be supersonic through the paper at 1000 yards in 60 degree F
temperatures at 600 feet altitude.  But a Sierra 30 caliber 180-gr. HPMK
leaving at 2610 (actual) will be supersonic through a 1000-yd. target at
sea level in 40 degree F temperatures.


From: (Bart Bobbitt)
Subject: Re: [RIFLE] Scope base for long range target shooting.
Organization: Hewlett-Packard Fort Collins Site

Most folks don't realize that standard scope bases/rings typically
align the center axis of the scope tube with the reciever/barrel axis.
This means the lower half of the scope's elevation adjustments can't
be used.

O.K. Weber, Eugene, Oregon has these bases in aluminum for about $60.
Phone 503-747-0458.

Most folks make these bases with about a 20 MOA front slope on them.
Jim Cloward (Washington state) makes the ones that OK Weber sells.

If you want to use an aperture sight on the rifle too, these bases have
a cut-out place on the back to let the rear sight base clear it.

Jack Davis (Monterey, California, makes these long bridge bases with
intregal rear sight base for about $100.  Phone 805-467-3765.

You may want to shim the base at different amounts at the front and
back where it screws into the reciever.  After installing the bridge
base, putting your scope on it, then getting a boresight, you can
determine how much to shim in thousandths to get the scope's elevation
about 2 MOA off the bottom when it's at boresight.  This gives you the
greatest amount of elevation adjustment usable.


From: (Bart Bobbitt)
Subject: Re: Need advice on Win. M70
Organization: Hewlett-Packard Fort Collins Site

Lloyd D Reid ( wrote:

: You will need just a bit over 35 minutes of elevation from your 100
: yard zero, to be on target at 1000 yards with a .308 Win.

Close, but no banana......

Depending on:

  * Altitude the rifle's shot at,

  * Specific bullet used, and

  * That bullet's muzzle velocity,

more than 45 MOA of elevation above boresight may be needed.  For example,
these Sierra HPMK bullets at 1000 ft. altitude require:

  * 155-gr @ 2900 fps, 40 MOA

  * 168-gr @ 2700 fps, 43 MOA.

  * 180-gr @ 2600 fps, 45 MOA.

  * 190-gr @ 2500 fps, 46 MOA

  * 200-gr @ 2400 fps, 49 MOA


Index Home About Blog