submitted by Paul Workman.
Based on an article by Rick Jamison of "Shooting Times" Magazine
Distortion of the scope tube (bending) can result in vertical or horizontal adjustments either sticking or not returning to the original set point after being adjusted for wind and elevation. For example, sometimes an adjustment or two will produce no change in POI, and a subsequent adjustment results in a huge jump in one or sometimes both the vertical and horizontal directions. Another example is the scope not returning to zero, even if the corrections are ultimately made in the same (“down and left” ) directions.
Scope tube bending can take place if the alignment of the rings is off by only a few thousandths of an inch. This can occur as result of errors in the rings or the height of the receiver between the rings. Further, if the axis of the rings are not in perfect alignment with relation to each other, the scope tube will be bent which can result in binding. Both of these situations can exist whether a one or two piece scope base is used, depending on the rings and the base(s) used…in theory anyway. And, there is some potential for distorting the tube by over-tightening the ring caps.
Lapping or epoxy can be used to correct the alignment problem.
Lapping has the advantage of being perhaps the fastest method from start to finish, but it must be done carefully and patiently to avoid creating more problems than the one intended to be solved. The lapping tools wear out and a worn lapp can result in less than perfectly straight and aligned surfaces for the scope to rest in. Constant and uniform pressure and elbow grease must be applied to a lapp in fresh & true condition. And, even with the best of lapping jobs, the actual fit to the scope tube is almost never perfect: The radius of the scope tube being less than the radius of the ring resulting from lapping – usually resulting in a two point hold when the scope tube is sandwiched between the bottom half of the ring and the top half. The result is less grip on the scope often leading the shooter to over torque the cap screws in order to assure the tube doesn’t slip under recoil. Thus the tube is distorted by taking on an oval at the point of ring contact; which may or may not cause a problem depending on degree of distortion and other factors.
Epoxy mounting solves the perfect fit problem. The epoxy exactly fills the gap between the bottom half of the rings and the body of the scope tube. The fit is similar to that resulting from glass bedding the action into the stock; complete and continuous. However, start to finish requires about 24 hours, depending on the epoxy used. One the other hand, the epoxy method requires no elbow grease.
The resin will do the lions share of the holding – far better than the rings alone, in my experience. The scope is now held in a mold making an exact fit – providing maximum contact. Thus the resin/epoxy mounting applies tremendous holding ability compared to the point contact holding that usually results even after lapping. This means less need to crank down on the top half of the scope rings, and thus lessening potential for distorting the scope due to over tightening the top half of the rings.
Not counting curing time, time spent aligning the rings and later the scope itself, the epoxy method requires less than 5 minutes actual effort. Not often is easier better, especially when it comes to accuracy.