I've talked about scope parallax many times over the years because it's such an important and misunderstood aspect of rifle performance. When people complain about the accuracy of their rifle, parallax is all too often the culprit. This makes understanding how to compensate for it an invaluable skill you'll need to get the most out of your rifle.
Before reading further, make sure to check out the TV spot I did for ShootingUSA on parallax. Due to time constraints, I was really only able to scratch the surface of the subject here, but it's very worthwhile to watch and is "required viewing" for what comes next.
Setting the parallax of your scope precisely is most important for benchrest shooting when you want to punch paper for groups, as you would for load development or an accuracy assessment of your rifle. When shooters have problems with consistency at these times, they've typically neglected to test for the existence of parallax or to adjust for it accordingly. They might also be shooting at a distance for which a non-adjustable scope is not set. In the end, the ugly truth is that the parallax must be reset every time you change the distance to the target if you really want to achieve maximum accuracy.
That being said, not every use of the rifle demands such meticulous adjustment. When we're talking about hunting accuracy requirements, having a precise setting on the parallax is really not critical. You're not likely to miss a game animal at any realistic hunting distance on account of it.
For field work like your typical hunting scenarios, the question to ask yourself is, "What distances am I most likely to make a shot at my intended target?" With that in mind, you can pick a compromise setting for the scope and just leave it set.
Let's say you expect your hunting shots to be between 200 and 400 yards. Given that, you could reasonably leave your parallax at 300 or 350 yards. I don't do a lot of hunting, but at one of the long-range precision events I shoot, most of the shots are in the 200-800 range. I'll usually set my parallax to 500-600 yards and not fidget with it any further.
While setup is one piece of the puzzle, you also need to know what to expect from the optic you're using. Always bear in mind that the yardage numbers on your scope may have no relationship to the actual setting. As a rule of thumb, the less you spend on a scope, the less likely that the yardage marked on the parallax setting will coincide with the live results.
Leopold, for instance, does not even mark yards on their scopes with side-turret parallax. This actually makes complete sense, as the appropriate setting will actually vary with the temperature of the scope. Just as I cover in the optics segment of GasGunBasics, you have to test your settings after you adjust the scope and just keep tweaking until you've nailed it down. There will never be a reliable shortcut.
The problem of parallax adjustment is actually most pronounced on non-adjustable, variable-power scopes in the 3-9x to 4-16x range. These are typically preset to be parallax-free at 100 yards since the manufacturers know that most people do their shooting on paper at that distance for zeroing and groups. The scopes are designed to be parallax-free at that range in order to produce the best results for that exact use. If they experience parallax error at 300 yards, a shooter will likely never know since they can probably still hit a deer-sized animal without issue.
However, if you take a non-adjustable scope and look at, say, a 400-yard target, the result will be much like what's shown in the ShootingUSA segment. Moving your head around while maintaining a stationary rifle, you will see the reticle move as much as six inches on the target. That's the possible error that you would add to the actual dispersion of the rifle at that distance.
Here's what I haven't yet covered in video: it is possible to mechanically eliminate parallax in any system by making sure your eyeball is in exactly the same position relative to the ocular of the scope. If your eyeball is in exactly the same place every time, then the reticle will maintain a constant position relative to the target, even if the scope is not parallax-free. You could call this "field expedient parallax elimination."
How do you do this? Over the years, experienced shooters will often develop a very constant cheek weld and head position on the stock that is very repeatable for them. I also know many shooters who make a point of placing the tip of their nose on the back of the charging handle on an AR-15 type rifle. This provides a registration point and ensures that your eyeball is always very close to a set location relative to the scope.
Less experienced shooters may need to discipline themselves by other methods. If there is no spot to place your nose, there's another technique you can master with an investment in practice.
First, back off the scope until you see a ring of black around the image. This signifies that you are slightly "out of the eye box" of the scope. If you move your head until the black ring appears to be equal all around the edge of the image, you have, in fact, placed your eye in the exact center of the bore of the scope. Standardize on this view, and you've solved the problem. Duplicating this view from shot to shot will actually eliminate the parallax error for a non-correctable scope or an adjustable scope that has not been precisely set.
In the future, I'll be drilling down on this subject even further in an expanded video presentation, but for now, I hope you get some good use out of this. Good shooting!