Sighting-In a Rifle to Shoot Different Loads
By Gary Zinn
Suppose one has a hunting rifle with which he/she wishes to shoot loads with different bullet weights and velocities. What are the principles and guidelines that may be used to sight in the rifle to seamlessly change from using one load to another? The answer, I believe, is both simpler and more complicated than one might imagine.
After thinking about this issue for some time and crunching some ballistic data, I came up with an example scenario that illustrates both the simplicity and complications of changing from one load to another.
Suppose that I have a .308 Winchester rifle, with a 22-inch barrel, and I want to be able to confidently shoot hunting loads with 150, 165, and 180 grain bullets. These loads are not going to shoot to the same points of impact at extended ranges, due to variations in the bullet weights and ballistic coefficients, along with the differences in muzzle velocities (MV) at which the bullets are driven. Here are some example data that illustrate the issues involved in shooting different loads from the same rifle.
MPBR and ballistic data for example loads
Consider the following .308 Winchester handloads:
These example loads are based on data in the Hornady Handbook of Cartridge Reloading (10th edition, 2016). MVs are for test loads fired in 22-inch barrels and are attained by several different powders and charge weights listed in the tables. The bullet is the Hornady flat-base InterLock Spire Point, a well proven big game bullet design. Other load specifications are listed on the first page of the .308 Winchester data section of the Hornady Handbook.
At Guns and Shooting Online, we believe the best way to sight-in hunting rifles is for their Maximum Point Blank Range (MPBR). Given the load specifications above, it is easy to use online ballistics programs to calculate the MPBR of each load, plus other relevant data.
Using the shooterscalculator.com Point Blank Range calculator yields the following results, with the program set for a +/- 3 inch MPBR, i.e., a 6 inch target diameter. (In addition to the MPBR, I show the Far Zero [FZ], Near Zero [NZ] and 100 yard sight-in elevation [100 S/I] for each load.)
From this, it might appear that if the rifle were sighted in for (say) the 150 grain load, then the 165 grain load should shoot very close to its indicated MPBR with the same sight setting. Meanwhile, the 180 grain load might fall slightly short of the indicated MPBR, because the optimal 100 yard sight-in elevation is 0.07 inch higher than that of the 150 grain bullet load. Further analysis will show whether this superficial conclusion is correct.
Trajectory analysis of the loads uses the Shooters Calculator Ballistic Trajectory calculator. The key variables, conventional (G1) bullet BC, bullet weight (grains) and MV (f.p.s.) are entered, along with the desired zero range (far zero). The program's default setting is for a sight height of 1.5 inches (a low mounted hunting scope with an objective of 40mm or less), which I did not change.
I set the "chart range" for 260 yards, which is just longer than my longest MPBR. The "chart step size" can be set to show external ballistics in 1, 5, 10, 20, 25, 50, or 100 yard increments. I set this to 1 yard for this analysis, although I usually do ballistics analyses for one of the larger increments, which makes for a smaller output table.
There are also changeable parameters for shooting angle, wind speed, wind angle and ambient shooting conditions (altitude, temperature, barometric pressure, humidity). I left these at their default values.
Here are the key results if the rifle is sighted in for a +/- 3 inch MPBR with the 150 grain load. The data is in the format: range in yards / bullet trajectory in inches.
150 grain load: 24 yds. / +0.01 in.; 100 yds. / +2.80 in.; 122 yds. / +3.0 in. (apogee); 200 yds. / +1.22 in.; 221 yds. / +0.02 in. (far zero); 259 yds. / -3.01 in. (MPBR)
Note that the trajectory calculator yields near zero and MPBR values one yard longer than indicated by the point blank range calculator. Such small differences pop-up from time to time between the two calculators, but are of no consequence.
With this data in hand, it would be a routine task to sight in a rifle for this load. I suggest a step-by-step procedure for Sighting In a Rifle for Maximum Point Blank Range in a companion article.
What will trajectories be if 165 or 180 grain loads are shot from the rifle with the sight set for MPBR with the 150 grain load? To evaluate this, I make the assumption that all three loads will have the same height of trajectory (elevation) at near zero distance (25 yards or less), but then the heavier, slower loads will lose height, relative to the 150 grain load, as the range lengthens.
I am comfortable with this assumption, because I have done the math for a variety of high-intensity cartridge / load sets. Generally, near zeros for +/- 3 inch MPBR analyses are at, or very close to, 25 yards downrange, while bullet elevation differences for different bullet weight loads in the same cartridge vary no more than a few hundredths of an inch at near zero range.
Applying the ballistic trajectory calculator under this assumption involves shortening the far zero values entered in the program, for both the 165 and 180 grain loads, to the point that 24 yard trajectory height will be 0.01 inch. Here are the results.
165 grain load: 24 yds. / +0.01 in.; 100 yds. / +2.65 in.; 120 yds. / +2.77 in. (apogee); 200 yds. / +0.61 in.; 210 yds. / +0.02 in. (far zero); 249 yds. / -3.02 in. (MPBR)
180 grain load: 24 yds. / +0.01 in.; 100 yds. / +2.49 in.; 113 yds. / +2.55 in. (apogee); 198 yds. / +0.01 in. (far zero); 237 yds. / -2.99 in. (MPBR)
The major result of this analysis is that the 165 and 180 grain loads will not shoot to an optimal +/- 3 inch MPBR with the sight zeroed for the 150 grain load. MPBR of the 165 and 180 grain loads will be 4 and 9 yards shorter than optimal, respectively. Meanwhile, the apogee of the 165 grain load will be about 1/4 inch below 3 inches, that of the 180 grain load nearly 1/2 inch below 3 inches.
What to do?
One is left, then, with some decisions to make regarding how to cope with the fact that all three loads will not shoot to optimal MPBR when the rifle is sighted-in for one of them. The simplest response is to sight-in for one of the loads and let the others fall where they may.
For instance, if I were mainly using the rifle to hunt deer, I would sight-in the rifle for the 150 grain load. I have used a .308 Winchester to hunt deer for many years, using 150 grain loads exclusively, with great success. The general principle would be to sight-in the rifle for the load that would be used most, then be aware of any meaningful difference in MPBR and apogee if another load were used.
Alternatively, one could split the difference among multiple loads. In this example, sight in the rifle for optimum MPBR with the 165 grain load, calculate (and range test) the trajectories of the 150 and 180 grain loads with that sight setting, and keep records on the differences in MPBR and trajectory, so you know what to expect when you switch from one load to another.
Another way of dealing with the different trajectories of the loads would be to change the elevation setting of the scope sight when one changes the load being used. What follows assumes, for sake of illustration, that the rifle sighted-in with the 150 grain load will shoot to exactly 2.80 inches elevation at 100 yards. (This assumes a scope with 100% accurate and repeatable adjustments, which hunting scopes very seldom have. -Editor)
Conformity of actual sight-in elevation to calculated elevation would be happenstance. The actual sight-in elevation would more likely be anywhere up to +/- 1/8 inch off of the calculated sight-in elevation, with a scope that adjusts in perfect 1/4 m.o.a. increments.
The trajectory calculator indicates that a 165 grain bullet will hit 2.65 inches high at 100 yards, when the rifle is sighted in for optimum MPBR with 150 grain loads. If the rifle is sighted in for optimum MPBR with 165 grain loads, the 100 yard sight-in elevation would be 2.82 inches. If one raises the elevation one click (0.25 inch at 100 yards), the adjustment would actually overshoot the optimal elevation change (.017 inch). The adjusted elevation setting would bring the 165 grain load closer to optimum, but it will not be right on.
Similarly, the 180 grain load is calculated to shoot to 2.49 inches of elevation at 100 yards, with the rifle sighted in for the 150 grain load. Meanwhile, the optimal 100 yard sight-in elevation for the 180 grain load is 2.87 inches, a difference of 0.38 inch between the 150 and 180 grain sight-in elevations. Should one adjust the scope one click (0.25 inch), or two (0.5 inch) in this case?
Issues with changing scope settings
I am not a fan of casually twirling the elevation or windage settings on a rifle scope, for several reasons. Basically, there is too much potential for making a mistake when one messes with a scope that has been sighted-in for a particular load. For instance, I have worked with scopes where the adjustment clicks were not crisp, so it was difficult to be sure how many increments of adjustment were being made.
As another example, suppose that I adjusted the elevation of a scope (say) three clicks when I switched from using one load to another that shoots on a significantly different trajectory, but then forget that I had made that adjustment when I subsequently switched back to the original load. I might not realize this oversight until I missed a shot at a game animal (or, worse, made a crippling hit), because the rifle was not shooting where I thought it should. I could go on, but I believe you get the idea.
I realize that so-called "tactical" rifle scopes, with easily accessible and readily adjustable turrets, have become somewhat popular on the shooting scene. To satisfy my own curiosity, I did a quick survey of these products, using the MidwayUSA website. MidwayUSA catalogs nearly 100 "tactical rifle scopes" (the search phrase I used), but most of these are not practical hunting scopes, because their magnification range is not right for hunting, their objective lenses are too large, they have cluttered reticles and the better brands and models are very expensive.
Only about one-third of the scopes listed had magnification ranges of 3x-9x, or lower. My view is that any big game hunting rifle that mounts a scope that has a low end magnification greater than 3x, and a high magnification greater than 9x, is wearing too much scope.
I say this for two reasons. First, most popular big game (Class 2 or 3) hunting cartridges have +/- 3 inch MPBR ranges that fall roughly between 250 and 300 yards, depending on the cartridge and load in question. A high end magnification of 9x is more than enough to get a clear sight picture on a deer or larger animal at those distances.
Second, it is likely that most commonly hunted game animals are taken at ranges of 100 yards or less. For these shorter range shots, a 2x to 4x magnification is more than adequate. My personal favorite deer rifle scopes are 1.5-5x or 2-7x, with objective lenses of no more than 33 mm diameter. (See Riflescopes for Hunting Class 2 Game for further thoughts on selecting a hunting scope.)
Most of the tactical rifle scopes I browsed have cluttered reticles, with things such as MOA dots or stadia marks on the cross hairs. A simple medium cross hair or duplex reticle, or one with an added center circle, works much better for game hunting scopes.
Conversely, a scope with MOA hashmarks, Mil-dots, or stadia lines on the crosshairs makes a certain sense on a varmint rifle that may be used to take extreme range (beyond MPBR) shots. The varmint hunter can use such reference markings to aid in estimating holdover for extreme range shots and in making hold-off adjustments in significant cross winds.
Scopes that have both some system of dots, lines, or tiny circles visible in the FOV and readily accessible and adjustable tactical turrets are, in my view, a redundancy. If you have one, why do you need the other?
If I were choosing a scope to mount on a varmint rifle that I expected to shoot a lot at extreme ranges, I would favor one that has MOA marks on the crosshairs, but not tactical turrets. The dots would help me make holdover and hold-off sighting adjustments and I would not be tempted to start spinning those turret dials.
Think about it, if one spent a day shooting over a prairie dog town, frequently adjusting turrets for different shot opportunities, the shooter will have no idea where the scope is pointing at the end of the day. To me, scopes with tactical turrets make no sense for either game or varmint hunting.
Another reason I am not enamored of tactical scopes is price. Fully one-half of the tactical scopes listed by MidwayUSA were priced from $500 up to $2400. Considering that comparable scopes, without the so-called tactical turrets and busy reticles, can be bought for roughly one-half to two-thirds of those prices, I cannot see the benefit of paying a high price for a scope on which I might change the elevation setting a couple times a season (if at all) and which has a poor reticle for hunting, to boot.
Additional thoughts and qualifications
I reported the trajectory data above to two decimal places, partly because that is the level of detail returned by the program I used, but also because I wanted to show the differences in trajectory between loads as precisely as possible. However, measurement of groups shot on target will normally not be that precise. I measure groups to 1/10th inch accuracy. Any elevation or windage data that are more detailed than that come about by averaging multiple shot groups.
A related topic, already mentioned in passing, is that it is a fortuitous accident when the 100 yard sight-in elevation of a particular load exactly matches the elevation calculated via a ballistic trajectory program. More likely is that the best sight-in elevation attainable may be as much as 1/8 inch higher or lower than the calculated elevation. The same applies to getting the windage of a load exactly on line. This is because most hunting scopes are designed to adjust elevation in 1/4 m.o.a. increments.
There are several other rifle, ammunition and shooter related variables that can frustrate any attempt to get a particular load to consistently shoot to a given point of impact at a particular range. Unfortunately, discussion of these is beyond the scope of this article.
Perhaps the most important additional thought I can share is that computer generated ballistic data should always be verified by actual shooting. It might be tempting to do a 100 yard sight-in with a particular load and then trust that where the ballistic program says that load, and others that might be used interchangeably, will hit at longer ranges is correct.
However, I am never fully confident of where my bullets will hit at extended ranges until I have shot some test groups. Generally the results are close to what the trajectory table indicates they should be, but occasionally test groups shot at 200 yards with the sight-in load will be off enough to prompt a one click adjustment (1/2 inch at 200 yards) in the elevation or windage of the scope.
The bottom line is that the only way to be sure how a particular rifle, load and shooter will interact to place bullets at any given distance is by shooting. There is no substitute for trigger time to gain confidence in how your rifle shoots particular loads at various ranges. This is especially important for the hunter, because shooting positions, accuracy and consistency in the field are very different from test firing from the shooting bench. The article The Personal Range Limit is especially instructive on this point.
It is probably clear where I stand on the issue of sighting-in a rifle when shooting significantly different loads. I favor MPBR sight-in for the load that I am likely to shoot the most, then calculating trajectories of and test firing other loads that I might occasionally use. Once I have verified how those other loads fly with the primary load sight-in, I write a note card summarizing the key ballistic information for each load and tuck it in the rifle case. This way I can quickly refresh my memory on how any load I use will perform. (Some hunters tape a note card with such information to their rifle stock, although I have never gone that far.)
I will not criticize those who choose to adjust their scope setting for different loads, if this is their preference. I do not go there, because I would rather keep things as simple as possible, even though it means I am not shooting loads, other than my primary one, to their optimum MPBR.
Copyright 2018 by Gary Zinn and/or chuckhawks.com. All rights reserved.