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Point-Of-Impact Shift

Point-Of-Impact Shift
Benchrest competition offers some scope innovations other precision shooters may apply.

Do 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.

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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?

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 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.

I will admit Benchrest is a very esoteric endeavor, and few riflemen expect teen aggs from their rifles. But there are times when POI shift can ruin a hunt or some other competitive sport. On two occasions I have slipped on a mountain side and taken a very bad fall. In both cases, the scope ended up between me and a rock during the fall. In one case, the tube body was badly dented. In another, I could not see that the scope had been harmed. But in both cases I checked it anyway, and, in both cases, the sight-in was off both vertically and horizontally, significantly enough to have missed any animal I might have shot at. One of them ended up 16 inches high and 12 inches left at 100 yards.

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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.

If you suspect unexplained, errant shots might be from a scope with POI shift, there are a couple of things you can do. Devise a method to hold the rifle down on bags so it does not move. Aim at a target and dial the turrets. If one or both do not respond or responds by jumping around, you probably have a bad or broken leaf spring or one that has moved out of position. I had it happen once. I sent the scope back. It was repaired with a note attached that said one of the leaf springs had not been heat-treated and had broken. Which brings up the second option. Just send the scope back to the manufacturer and tell them the scope will not hold point-of-impact. Many will replace the scope or the internals if that is the problem.

Some people, including some writers, like to box a scope. That is, shoot at a point, dial the turrets up some arbitrary amount, then right, then down, and then left back to the beginning point and see if that shot goes in or near the same hole. Really enthused boxers go around the box several times to see if the dials are repeatable. Others wonder if the dials really give them 1/4-inch moves per click, or even if some clicks do and some don’t. Of course that might be MOA, centimeters, MILS or whatever. The process of boxing with live ammo might tell you if the movement is reliable under recoil, but it can be done without shooting as well. Hold the rifle down, put the crosshair on an intersection on a grid, then move up, right, down, and then left. Determine if the crosshair moves as advertised and reliably. I did on a suspected scope and found out immediately that the windage turret was not moving the crosshair at all or in jumps.

A scope manufacturer at a writer’s conference once said the average, reliable life of a hunting scope was 3,000 rounds. After that you could expect POI shift and other problems. For the average hunter, that might mean a lifetime of shooting. For others, that would mean less than a year. Most benchrest shooters have found that premium match-grade barrels are good for about 2,000 rounds using 6mm PPC ammo. At some point, a shooter is going to be frustrated by either his scope or his barrel and wonder which is the primary culprit.

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A newer rendition of the old posi mounts. This arrangement is frequently used by very long-range varmint hunters, although the rear mount would be extremely high. Sight-in changes are made by turning the micrometer dials for both elevation and windage. However, extreme sight-in changes may put stress on the scope body through the ring. Also, where the micrometer touches the scope body, a wear point may begin to form.
By Jacob Gottfredson

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