I have read a few posts on here mentioning that a scope must be shimmed to get enough adjustment for long range on some caliber/setups. I am shooting a 30-06 plain old stock rifle using a cheap 6-24x Tasco. I plan to move to a Nikon soon. My question is three fold . How do I know if a particular scope will need shimmed? Do they all need it for this caliber? The last question is about the alignment of the scope. If it is canted one way or the other, it would create horizontal movement when adjusted up or down. How can I tell if it was mounted correctly? The only real option I have is a larger gun store. They have on occasion bore sighted rifles that barely hit a 4'x4' target at 100yds, so I lack faith.
1. How do I know if a particular scope will need shimmed? A scope needs to tilted downward in the front if you run out of elevation adjustment when you try to center your scope on the point of impact of the longest range target you want to shoot. You call that shimed, but it's normally accomplished by replacing the scope base with one which has a wedge shape machined in.
many manufactures make wedged bases for various rifles. 20 MOA is common but I've seen 5, 10, 20, 30, 40, and 60. I use use them on several of my rifles.
You can calculate the angle you need, but you need a ballistics program and many of the details of your rifle, ammo, and atmospheric conditions. You also need the specs on your scope, specifically it vertical adjustment range. Very briefly, if you don't use a wedged base (or shim as you call it) the maximum distance you can zero will have a drop in MOA of about 45 % of the total MOA range of the scope. If you use a wedged base you should be able to use over 90% of the total elevation range of the scope. A wedged base can roughly double the useful drop in MOA, though hat will be less than about 1.4 times the distance you'll achieve without a wedge. If you want to shoot long range select a scope with has a lot of vertical adjustment. You mention wanting a Nikon but you didn't say what model. Picking a scope with generous vertical adjustment does not need to be expensive. Any scope I consider for long range will have at least 100 MOA of vertical adjustment. I have several with 140 MOA, and still use wedged bases for extreme range shooting. That's not for hunting.
2. Do they all need it for this caliber? No, That is depndent on the distance you choose to shoot, the ammo you use, the atmosphere where you shoot, and the adjustment range of the scope you choose. A 30-06 can shoot well over 1000 yards supersonic with the right bullets, but that's well beyond it's practical hunting range. How far you want to shoot is your decision.
3.How can I tell if it was mounted correctly? A scope should be mounted so it's comfortably centered in front of you eye when you're in a comfortable shooting position. What's right for one person in not necessarily right for another. It should allow a comfortable "cheek weld" to the rifle stock so you can assume the shooting position easily. The eye relief should be comfortable and enough that your eyebrow or glasses aren't hit by the scope. That's determined as much by how you hold and control the rifle than the caliber or spacing. The line of the elevation adjustment axis should be parallel to a line though the center of the bore vertically though the centerline of the scope. If the scope is properly made the vertical reticle (if there is one) should be parallel to that line too, though the adjstment axis is more important. Use the same tools as for construction when mounting a scope. Squares, and levels are useful. So is a padded vise to hold the rifle. There are many ways to align it which work.
To achieve decent accuracy all of the mating surfaces of the action, base,rings, and scope must fit without warping when the screws are tightened. Surfaces can be lapped or bedded when the fit isn't good.
A rifle will not give good long range accuracy if it's not held vertical for each shot. That's called canting. Many long range shooters use a bubble level attached to their scope for every shot. In normal condition most people are fairly good at sensing vertical, but in wind and stormy conditions where there are few natural indicators it's more difficult.
