Mounting a scope on a rifle can take several forms. You can just bolt it on, hope for the best and ping away like most people. Or you can take serious steps to ensure that your scope is properly aligned within the mounts to minimise refraction error, properly align the rings to avoid denting your scope tube, and even lap the rings to give better ring-to-tube fit. I am not an authority on the subject, and my termination may not be always correct, but my experience of fitting scopes over the past twelve years has taught me many tricks of the trade, some of which are included here.
None of the mounts listed below are considered high cost. I am only concerned with mounts that the average shooter can afford, so don't get offended that I haven't mentioned your Buehlers.
1. Sportsmatch (England). While made from not-so-ideal aluminium, the design of these mounts is excellent. All heights are made, including a one-piece bridge-style, which is solid as a rock. Their most endearing qualities are using large allen screws, and having two locking screws per dovetail clamp and four per ring. Made mainly for high powered air rifles, they hang on like crazy.
2. Tasco World Class. These mounts used to be great, until the dovetail clamp design was cheapened. Made from steel, they certainly are strong, but their design lacks a certain something to make them the best.
3. Pioneer. Not available any more, these were the ultimate in strength. Made from solid blocks of steel (that significantly added to the rifle's weight), they suffered from poor quality control at the factory (ill fitting on the dovetail, poor finish), but boy, if they fit, nothing will shift them.
4. Zero. Alloy body with steel clamps either side. Has the advantage of being windage adjustable (screws either side of dovetail clamps). Has the disadvantage that if you overtighten the phillips head screws the steel clamps will bend and raise the level of the ring. Mounts are then stuffed.
5. Chinese Junkies. Various brand names, they suit the casual shooter. All alloy, with one side clamp. They work okay.
1. Weaver. Consists of alloy base/bases screwed to the receiver, with an alloy bodied ring base and spring steel tube clamp. The real beauty of these mounts is the way they lock together. The base is a dovetail with a slot cut across; the ring base is clamped to the dovetail with an exposed screw which locates in the slot, thereby locking solidly in place. The spring steel clamp really grips also. Care must be taken when pushing the clamp over the scope tube, it can scratch the finish off an alloy tube. I often put one layer of sellotape around the tube where the clamp sits to protect it. But do not overlap the tape.
Apart from the design strength, I like the Weaver because it is possible to seat the scope low on the receiver; many steel bridge mounts are way too high, even with low rings. They do look ugly, but they work.
2. Tasco World Class Bridge (New Model). This is a distant second, as there are many things I do not like about these mounts. They are steel, which is good. They have windage adjustment, which is also good. The piddly screws which adjust windage and hold the rings in place are not so good, neither is the height factor on many rifles.
3. The Redfield System. Made by Redfield, Leupold, early Tasco World Class and others. Features a front ring with a cam-in lug onto the front base. Do not regularly remove and replace this mount, as every time you do so it loses tightness in its fitting. Rear ring is held in place by the top fraction of two locking screws on the rear base, which gives it windage adjustment. Strength at this point is pathetic. How they ever sold so many of these mounts is way beyond me.
4. Parker Hale. Not rated lowly for any reason except they are available for a limited number of rifles. All alloy, the design is excellent, although has no provision for windage adjustment. Rings locate onto a dovetail, with at least one ring having a locking pin which locates in the base.
It is a good idea to ensure your bases do not come adrift by using some form of locking fluid on the screws which hold the base to the action. This is common sense. But don't be tempted to use a high grade Locktite - many of these can only be loosened with an oxy set. A low-grade Locktite is adequate, otherwise pinch your wife's nail varnish for the job.
Dovetail mounts with no windage adjustment give you no choice, you simply put them on and tighten the screw(s). Windage adjustable mounts are a different story. In order to do as little damage as possible to the scope, I use a steel bar to position the rings before tightening them on the base. This aligns the rings and prevents them from kinking the scope tube.
My steel bar has two uses, and therefore is not your average steel bar. I took an 18" length of precision ground high tensile 4140 grade 1" bar and fitted a handle to each end. I don't suggest you go to that expense, but it would help if the bar is within a few thou of 1", and has enough length to overhang the rings by six inches either end.
If you place the bar in the rings and tighten the ring caps you will have aligned the rings. Now tighten the rings on the base(s). If you keep the screws even on either side you will require minimal windage adjustment. Now remove the bar and start fitting your scope.
The more meticulous shooter might wish to lap the rings to increase bearing surface on the scope tube. In this case, after aligning the rings as above, take the ring caps off and leave the steel bar in place. Smear a small amount of fine grade water based grinding paste around the bar. Move the bar in a figure-of-eight backwards-forwards motion for a couple of minutes. This will lap the ring bases and improve contact surface on the scope tube. When finished, remove all traces of grinding paste and install scope.
Most budget priced variable scopes will prove to be variable in more ways than one. When moving the power setting up or down, the point of impact on the target will also move. With image moving scopes, where the crosshairs remain apparently centered in the field of view, the crosshair itself is moved within the tube in order to correspond with where your bullets hit. This is what happens when you turn the micro adjusting knobs for windage and elevation. A fixed power scope can cope with this misalignment, but a variable often cannot.
In a perfect world, the lenses that are adjusted by turning the power setting ring would remain in perfect alignment throughout its adjustment. Bad luck, this is not a perfect world, and the cheaper the scope the less likelihood of good alignment between lenses. So when they are rotated, they refract the light, with the result of your point of impact changing. Sometimes drastically. I once owned a Weaver 3-9 which printed a neat "S" shaped string 6" high on the target at 50 yards as I wound the power settings higher.
In order to minimise this, you must centralise the windage and elevation adjustments. This may mean counting the total number of clicks from one side to the other, then counting back half. Then you must sight the rifle in as best you can using the windage adjustment on your scope mounts.
Height is more of a problem. It is possible to shim one or both rings, top or bottom, by the thickness of one aluminium drink can shim (about .006"), without doing too much damage. Be careful when tightening the ring caps, and make sure the shims are a neat fit.
Remember, it is not necessary to sight it in perfectly in this manner, keeping the scope adjustments as close as possible to their centre will optimise your scope's performance.