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Borescope (Collimnator Type) Tips

This article was edited from software documentation to encourage a discussion about optical boresights and sighting methods. If you are an accomplished gun smith or old-hand enthusiast, much of what is in this article may not be new.

I continually run into shooters who think the only use for an optical boresight is to "get on the paper" after installing a new scope. No optical boresight will actually "zero" a firearm. Optical parallax due to variations in mounting height and differences in firearm trajectories makes this impossible by simply lining up the X's. Besides, on bolt action rifles nearly the same thing can be accomplished by removing the bolt and looking down the bore at a distant object, then comparing it to the scope's view. I know an accuracy expert at a well known company that insists all optical boresights are useless and prefers the "through-the-barrel" method or boresights that shine a laser through the barrel. Then what good are they?

I use my optical boresight to record and duplicate scope settings for different loads, ranges and shooting conditions with reasonable accuracy. I even move scopes between firearms without "re-zeroing";...and it works! My boresight has paid for itself many times over with saved time and ammunition. The keys to making it work is to keep meticulous records of boresight readings for each zeroed firearm, ammo batch and your favorite shooting places; and to use a boresight that always gives the same reading. Unfortunately there are a few boresights that will not provide the same reading twice in a row. This is probably why optical boresights often get a bad rap, so don't be cheap. Use a good one.

If you use your boresight to record scope settings, rather than just to find a rough zero, you can reproduce scope settings for any ammo, trajectory and shooting area before you leave home. Rarely am I off by more than a click or two when I get to where I am going to shoot so I can spend time practicing or hunting, not finding the target again.

I have found the trick to getting a boresight to work reasonably well when changing scopes is to first properly install the scope so parallax error due to mounting differences is minimized. Scopes are shipped from the factory with the reticle centered in the sight tube. Smithies all know simply slapping a new scope on "store-bought" mounts may result in a poor installation. Unfortunately lots of shooters mount their own scopes assuming everything should fit without modification. Sometimes it does; often it does not leaving the shooter wondering why he can't get tight groups or blaming the scope.

A scope should always be mounted so under normal shooting conditions the reticle remains near the optical center. This is especially true of inexpensive scopes. If the reticle is moved excessively off-center, the windage or elevation calibration may become inaccurate. This is why expensive bench rest scopes are custom made so the reticle is centered for a particular rifle mounting and zero range. Competition shooters want their scopes to move the impact point a known distance with each click through a wide range.

The traditional way of mounting a new scope is to first attach the bottom half of the rings to the rifle and with the rifle on a good steady rest, sight through the bore at a target and then through the scope while it is resting on the rings. If the mounts are parallel with the bore then the scope should need only minor adjustment to move the reticle onto the target. More than a few clicks in any direction indicates the mounts need to be shimmed or reshaped. After shimming or shaping the mounts most good smiths will also lap the rings to ensure they are aligned and will not bend the scope tube when tightened. Even slight bending of the tube may cause the scope's adjustment mechanism to bind. Once this is completed, the scope is tightened in place and checked again to verify the scope is still parallel with the bore. Only after proper alignment are the windage and elevation adjustments moved. This method will provide good results, but does not ensure the reticle will remain in the optical center after the arm is zeroed for its trajectory.

An optical boresight comes in real handy on firearms where it is not possible to look through the bore, or to keep the reticle centered when zeroed for normal shooting conditions. But this does not mean simply clicking elevation and windage until the X's line up! If the boresight is new, always first confirm its grid is near "zero" by checking it on another sighted firearm of the same caliber and type of mounting. Note the point where the reticle falls on the grid. This point is usually a much better indication of the probable "zero" setting than the boresight grid's center or a view through the barrel. As above, if more than a few clicks in any direction are required to put the reticle on the proper grid point, then the mounts may need work.

You can check to see if a scope's reticle is centered by rotating it in the mounts while looking through it at a boresight grid or distant object. If the reticle moves in a large circle relative to the grid or object, then the reticle is not centered and your mounts probably need to be shimmed to keep it centered.

By keeping the reticle close to the optical center you will discover windage and elevation adjustments are more accurate over a greater adjustment range and the scope may seem to shoot better groups. You should especially notice the difference with inexpensive scopes. Any reduction of optical parallax means changes to eye position will have less effect on where the bullet hits. Another benefit is differences in mounting height will have less effect on boresight readings, so it is easier to compare settings between firearms.

Most smiths agree quality mounts are worth the investment. I prefer twist-off, one-piece Leupold or Redfield mounts with the built in windage adjustment. If you intend to move a scope between firearms the built in windage adjustment will allow you to compensate for windage differences in the mount, not the scope, so you may only need minor elevation changes to sight "dead-on" again. I change Leupold and Redfield scopes between a Remington 742 and 700 (with one-piece mounts) and it takes no more than 3 elevation clicks to get the scopes back on a perfect zero.

Here's some tips submitted by a gunsmith who visited this site.

A Collimnator (proper name for an optical boresight) can be used to obtain even bearing pressure at tip of fore-end in rifles fitted with a scope. Procedure is as follows:

(1) Remove barreled action from stock, insert collimator in barrel muzzle, and adjust telescope recticle until it is centered on collimator recticle.
(2) Replace barreled action in stock and draw up guard screws. If the barrel is bearing harder on one side than on the other....collimator reticle will be displaced in relation to scope reticle. Typical movement will be up and right or up and left.

Bearing must be adjusted by trial until reticles match center. I find just a very small amount of bearing straight up will put APROX. 5lbs. of pressure on pressure-point bedded barrels . The center of collimnator reticle will be directly above center of telescope reticle by a very small amount. Knowing the right amount will come with experience.

The right amount of straight up front bearing pressure can be measured with an ordinary spring scale. Clamp the fore-end of the rifle in a vise and then insert a piece of cellophane between barrel and bearing. Hook the spring scale to the barrel and pull the scale upward and away from the barrel with one hand while pulling on the cellophane with the other. The scale reading noted when the cellophane is released is the bearing pressure indicated in pounds.

Using a collimator on Free-Floated barrels. Uneven pressure on receiver will also be noticed through reticle displacement. Adjust bearing pressure on receiver until center of collimator reticle and scope reticle match exactly. Well that's how I do it and as the saying goes IT WORKS FOR ME!!

Gun Smithing Dave


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