Instruction manual for 1970's custom .22 scope with BDC
I recently bought a Bushnell Custom 3x-7x .22 scope from the 1970's. It has the Bullet Drop Compensating feature. I need a manual. How do I sight this little scope in for elevation? There seems to be no way to raise the elevation separate from the BDC feature...
Re: Instruction manual for 1970's custom .22 scope with...
Hold the outer portion of the elevation knob and unscrew the screw on top. then pull up on the outer portion and you'll see a silver screw marked with an arrow and the word "Up". Sight the rifle at a known distance and then line up the knob with that distance as you push it down on the scope and replace the screw. I have one of these and figured this out on my own, however I'm missing the mount and would like some pictures and dimensions of the mount if any of you can get some. I'm gonna be machining a new one and would like to keep it as close to the original as possible.
Re: Instruction manual for 1970's custom .22 scope with...
It would be nice to know what muzzle velocity the BDC is set for, but I assume it's standard high velocity, not Mini Mags and etc.
The one I have mounted on a Henry lever action has a double sided clamp that fits the groove on the action/receiver and the groove on the scope. Rings won't work due to the groove on the bottom of the scope.
The clamp mounts for lasers, like a BSA that has mounts for .22 grooved receivers and Weaver bases, would probably get your scope functional for $30 or so, and still have the laser that would work with Weaver bases.
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Take a rifle with a properly installed BDC bullet drop
compensating reticle to a rifle range. Using ear protection zero the
scope reticle so that the point of impact corresponds to the center of
the cross hairs at the distance you have selected - typically 100 or 200
Determine the trajectory of the specific cartridge you have
selected. There are several ways to do this. Ammo manufacturers publish
trajectory and wind drift information. There are web based ballistic
calculators like http://www.biggameinfo.com/BalCalc.aspx which will tell
you how much your bullet drops at known distances.
Scope manufacturers like Leupold can be a valuable source of
information about their reticle: "Leupold® Ballistic Aiming System:
Boone and Crockett Club® Big Game Reticle aiming system provides a
series of additional aiming points to improve your ability to shoot
accurately at longer ranges. Nikon
also provides good information suggesting the marks on their reticle be
used for zero at 100yds followed by circles below representing 200, 300,
400 and 500 yds if the cartridge travels around 2800 ft per sec. Nikon
suggests the center cross hair be zeroed at 200 yds for magnum calibers
traveling around 3000 ft per sec. We understant that each variation of
different bullet weight and powder charge changes trajectory and a scope
manufacturer can not build a different reticle for each different
cartridge made so practice on the range to determine how well the marks
relate to the actual impact of where your bullet strikes at a know
distance is important. The one thing that people using BDC scopes
typically have problems with is that a BDC scope has the reticle in the
second focal plane of the scope. If the reticle was in the first focal
plane of the scope the reticle would look smaller on low powers like 3x
and grow proportionately larger as the power increased to say 9x top
power. The problem is that while the marks on the BDC reticle correspond
accurately to the bullet drop at the know distances 200, 300yds etc.
What happens when you lower the power from the scopes maximum power to
any other lower power is the reticle stays the same size and the field
of view within the scope increases which means that the distance between
these marks on the BDC reticle no longer corresponds to the point where
the bullet will strike. In short BDC reticles only work at the maximum
power of the scope or at a set specific power. At all other powers these
BDC reticles do not accurately represent where the bullet will strike.
The center X always remains the same. If you zero at 100
yards and you know that your bullet drops 8 inches at 300 yards you
could forget about the BDC marks and hold the center X 8 inches high -
that works at any power 3x or 9x and should be used at lower powers. If
you zero the center crosshair at 100yds and have the BDC scope at the
maximum power 9X then the first line or circle below the center X should
be the mark you place on the center of the 200 yard target----- the
bullet strike should hit the center. If by some chance you put the scope
on 3x and placed that first mark below the center cross hair on that
200 yard target you would shoot over the top of the target. This is
because as the power of the scope decreases the field of view increases
the angle increase and gets wider. You can experiment with known power
settings and see at a specific power say 3x what that first circle down
corresponds to and make notes because at any set power what the marks
correspond to will be repeatable.
! click is 1 inch @ 100 yards, provided you have the correct scope for the rifle.
If you are sighting in a .22 LR 40 grain match velocity bullet for 100 yards, the mid range trajectory (50 Yards) is 6" higher. The bullet has a 6" arc half way to your target. If you are sighting in a .270 with 150 grain blunt tail bullets for 250 yards, the mid range trajectory is .4 higher. It only has 4/10" arc half way.
You most match the scope to the gun, and the 1 click= 1 inch rule will apply. Otherwise, it's lock it into a righd mount, fire it and adjust the scope accordingly. If you want to change the range on the scope, it will be guesswork.
I use a Bushnell Banner 3-9X40 with BDC. If you have BDC, make sure you have the correct distance ring in for the caliber.. It will make all the difference in the world.