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GUNS Magazine April 2013 Digital Edition - Page 12
Point-of-imPact SHift benchrest competition offers some scope innovations other precision shooters may apply. Jacob GottfreDson Bukys scope and mount shown from the right side. Note the turrets have been removed. The external adjustments do not ride on the scope body or transfer any stresses from the rings to the scope body like the old external adjusting posi mounts. D o riflescopes shift point-of-impact and for what reasons? They are notorious for doing so, but how much matters? That depends on what you are trying to do with it, i.e. what level of accuracy and precision you require. If it is great enough, it can affect any shooting endeavor. I have been competing in several shooting genres my entire adult life. None are so demanding of precision than 100-, 200- and 300-yard Benchrest. If, on a good day, you are not shooting 25-round aggregates in the teens, you are not competitive. If your scope shifts point-of-impact for any reason, you will end up in the bottom half or worse at the end of the match. The scopes used for many years were the 36X fixed-power variety made particularly for Benchrest. Shooters began to suspect that point-of-impact (POI) shift might be the reason they were not doing as well as they thought or hoped they should be. In the mid-’90s, Cecil Tucker from Odessa, Texas, began working on the problem. He found the erector tube was supported against the elevation and windage turrets by weak leaf springs positioned opposite the turrets. They exerted about 7 pounds force on the erector tube but did not push the erector tube back to battery efficiently. He also found the method used to secure the erector tube axially was inefficient as well. Those scopes also used an adjustable objective to adjust parallax that sometimes did not move as intended. He set about trying to correct these problems. He first removed the leaf springs. He then drilled a hole in the side of the scope body 120 degrees from the elevation and windage turrets. He placed a strong spring that exerted approximately 30 pounds of pressure against the erector tube to keep it in place. He also added a wave washer at the end of the erector tube to support it axially. The spring on the side of the scope was housed in a cylindrical tube, attached to the side of the scope. The strong spring supposedly ensured the erector tube was always held tightly against the turrets, hopefully preventing POI shift. A few years later Burris incorporated that idea in their scopes, calling it the Posi Lok. Burris added another twist. You could leave the spring to act just as Cecil’s did, or the cylindrical piece on the outside of the scope The internals of this scope have been modified and hard supported inside, the internal adjustments have been removed, and the adjustments transferred to an external mount. The device is shown on the left side. The owner, Gene Bukys, wins most of the matches he attends. Is this the reason? could be screwed in until the erector tube could be held firmly in place. To change sight-in, you had to unscrew the cylindrical piece, make the adjustment, and then screw it in again, or just use the strong spring. A few years after that, other ambitious and innovative shooters began taking scopes apart, understanding their mechanisms, and securing the internal parts to alleviate the problem. They were not satisfied with holding the erector tube in place with a strong spring alone. They also wanted to ensure the erector tube could not move in any direction and neither could the lenses. But this presented another problem: How were they going to change sight-in. The problem with the old mounts like those used on Unertl external adjustment scopes is the adjustments rode on the outside of the scope tube. These were not reliable either because the contact points wore uneven slots in the scope body. Probably the premier innovator is Gene Bukys. Besides securing the internals, he designed a mount that will move the scope for sight-in without anything touching the scope body. Did he succeed? No way of telling, but his performance might suggest he did. He wins most of the matches he attends, to include the National and World Championships. Could he have done it otherwise? Who knows? But one thing is certain, it has not hurt him. Another innovative top flight Benchrest shooter was fixing scopes at a slight cost by inserting a plastic cylinder in the side of the scope tube to hold the erector tube in place. All this caused some scope manufactures to look closely at the problem and begin to eliminate the problem of POI shift. The March scope was one such endeavor. That scope will be the subject of another article. 12 W W W. G U N S M AG A Z I N E . C O M • A P R I L 2 0 1 3