If you have a scope sight that’s calibrated in milliradians and one click is supposed to equal 0.1 mil of reticle movement, then five clicks should move the point of bullet impact 5 centimeters at 100 meters. Likewise, if it’s calibrated so that one click is supposed to equal 1/4 minute of angle, then four clicks should move the POI one MOA (1.047") at 100 yards. But do they?

Many long range shooting authorities tell us that those click/POI shift ratios aren’t always exact, and therefore we should test our scopes to determine what the actual adjustment values are. Before going further, I will point out that the question matters only in certain situations. If you zero your hunting rifle and adjust your point of aim for targets at different distances, if you dial your holds and all your shooting is close enough that minor inaccuracies don’t amount to much, or if all your long range shooting is at specific known distances and through live fire experimenting you know exactly what sight settings are necessary for the targets, then none of this matters to you.

If, however, you engage in long range shooting at targets located at different ranges, and you don’t adjust your point of aim but rather dial adjustments based on a ballistics calculator when engaging the targets, then this might matter.

For discussion, let’s say that when you dial your MRAD scope ten 0.1 mil clicks, it moves the point of impact by only 0.9 mils, or 90 percent of what it should. If the target is at 200 yards and called-for adjustment is 0.4 mils, but the POI is moved only 2.6 inches rather than the 2.9 inches calculated, then we probably won’t even notice the difference. What if, however, we’re shooting at 700 yards and our ballistic solver tells us the adjustment from a 100 yard zero is 4.4 mils (44 clicks)? If our scope moves the reticle only 90% of what it should and therefore 44 clicks equals ~3.96 mils, then even without taking into account the imprecision of the shooter, ammunition, and rifle, the bullet would hit about 10 inches lower than it should.

That is therefore why tracking accuracy is important for long range shooting. How then do we determine how accurate the adjustments of our scope are? The most common advice is to conduct a “tall target” test of the scope, and that brings me to the primary point of this post.

A tall target test consists of putting … ahem, a tall target at a known distance like 100 yards for minute of angle tests or at 100 meters for milliradian tests. We start by firing a group with our scope zeroed for the target distance. Then we dial the elevation up some convenient amount and fire another group. It’s often suggested that a MRAD scope be dialed up 10 clicks (1 mil) for each new group, and the group consist of at least three shots. Or we could do something like dial up 100 clicks (1 meter at 100 meters) and fire just one confirming group there. Once we have our groups, we measure how far apart they are. For the milliradian test, they should be exactly 10 centimeters apart as they go up. But that brings us to the drawback of the target test: how do we know where “exactly” is?

Anyone who has read this far no doubt already knows that no rifle and ammunition and shooter always put even three shots, much less five, through the same hole every time at 100 or so yards; in fact that’s exceedingly rare. I recently saw a tall target test on the ’net. The shooter claimed that his rifle was capable of 1/2 MOA groups “all the time”; if true, that’s a level of precision that most shooters only dream of. I’m a little skeptical of that claim based on the ammunition and rifle he was using, but assume it was true. That means that even within the claim, shots could be 1/4 MOA from the point of aim, or one full 1/4 MOA scope click. If the three (or more) shots cluster a bit above a point of aim, how do we know if it was due to a scope tracking error or the random distribution of the shots?

It’s true that if we shoot enough groups at various elevation settings, the average will probably give us a good idea if the scope is tracking properly, but if it’s not, then how much is it off? And of course, what if the precision level is only one full MOA; then the random distribution could be as much as 1/2 MOA from the point of aim.

The other way to check tracking is what I did with five of my milliradian calibrated scopes today. I prepared a series of 10 lines exactly 10 centimeters apart (used calipers to measure). I then placed the target a carefully rangefinder-measured 100 meters from the shooting position. Starting from the top line and dialing up 10 clicks at a time (the reticle moves down), I monitored how closely 10 clicks moved the reticle ten centimeters, and then finally whether 100 clicks moved it down to the line one meter below the start. As it turned out, all five scopes tracked as perfectly as I could detect by eye: 100 clicks = 1 meter of movement at 100 meters.
In the process I learned a few things.

The obvious difficulty with the method I used is keeping the scope and rifle absolutely still as the elevation adjustment is turned up. The more distinct and positive the adjustment clicks are, the more force is required to turn the knob, and the greater chance of moving something in addition to the adjustments.

The best method I found to ensure that nothing other than the adjustment knob and reticle had moved during the process was to dial back down to 0 after reaching the final line. At that point the reticle should be perfectly centered on the starting line. If it’s off, time to repeat the experiment, and try harder to not move the gun/scope. The first two scopes I tested were my most expensive models, and I was a little disappointed when it was necessary to dial only 99 clicks to reach 1 meter of movement at 100 meters. There was another shooter down the line that made it necessary to wear hearing protection at first which made it difficult to get into proper viewing position behind the scope without moving the gun at all. After he left I repeated the tests of those scopes several times and then 100 clicks = 1 meter of reticle movement.

The maximum magnification of the scopes ranged from 15 to 28×. I couldn’t always see the lines themselves, and anticipating that I put square brown stickers on the lines with points left/right and up/down. When viewing the stickers I was able to clearly see where the crosshairs of the reticles were positioned, and whether they were centered on the diamond shape.

As I say, the concern I had about the method I used was to keep the scope immobile. I clamped the rifles in a CTK Precision gun vise and although not absolutely rigid, it worked well enough. The other advantage of the vise was that it permits easy adjustment of the rifle for leveling and precise aiming at the starting point on the target.

I think I get the gist of what you're talking about but correct me if I'm wrong.

I call it a "Box Test" and I don't know if that's just a term or if it's really called that but I'll explain what I did for clarification.

I bought an older Winchester heavy barreled varmint rifle in 243 caliber and I mounted a Leupold fixed 20x scope to it. I can't remember the model of Leupold though and I'm to lazy to open the safe right now to look.

Basically, I zeroed the rifle to 100 yards and I wanted to see how the scope tracked doing windage and elevation adjustments. My friend who has a range out to 600 yards told me about this test and it is just making adjustments after making a group. So many clicks with windage and elevation so you make a box around the group you shot. You finally end up on your zero again and shoot another group to verify.

The ARs, I use hold over/under for distance but with the Winchester, I do adjustments on the scope itself. I have not shot it to 600 yards though and the farthest I shot that rifle is about 350 yards. With my load, 350 yards is about 19.5" drop.

Is this kind of what you're talking about or am I way in left field?

I've done the tall target shooting test a few times, when I've struggled with POI issues. I've done cursory "tall lines" type testing, but only with a stable bipod and an heavy sand bunny-ear rear bag. I was more confident with the results from the tall target shooting tests at 100 yards than by dialing up from a sorta-kinda stable rifle position.

I believe I discussed one tall target test a few years back in the precision rifle thread. This was shortly after I retired my first AR15 barrel, then switched scopes to a Vortex Viper PST II 2-10x scope. The POA/POI issue arose when I tested the Vortex for maximum up turret revs. When I went back to zero on the elevation turret, the zero changed. I tested up elevation to 40 MOA on a tall target. POI at 100 yards was almost perfect for dialing up 20, 30, and 40 MOA -- so the scope tracked. The problem was that when I bumped up hard against max elevation on the turret, the zero changed by 2-3 MOA.

IIRC, there was 50-55 MOA up travel before the turret topped out. I believe 40 MOA from the turret got me to around 1,000 yards, which is well beyond the capabilities of a 16" AR15. Probably would never shoot this AR past 700 yards, which is likely around 20 MOA of elevation. So, unlike my better Nightforce scopes, my mid-level Vortex should never bump up against the dialed elevation & windage extremes. I'm OK with that, given the price and performance of the Vortex. It's a nice scope.

I was not, and am still not, capable of regularly producing bug-hole groups at 100 yards with any AR. IIRC my 100-yard tall target groups were about 1/2" to 3/4", and pretty consistently round. Good enough for using the center point for measuring POI distances above zero. I was also concerned that this replacement barrel was not installed correctly and/or there was something wonky with the way the rail was installed. I took the upper to my long-time 'smith, who just left the store north of Denver known by MH (sigfreund, you know the place). My 'smith confirmed that MH didn't install either correctly. My 'smith said the issues were quite minor, and he found it interesting that I could detect such a minor problem. After my 'smith re-installed both the barrel and the rail, this upper now shoots wunderbar.

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Years ago I saw the post-shoot write-up from an ELR course co-held by Frank Galli and the top rifle instructor Gunsight Academy, in Arizona. Given that the distances of 2000 to 2500+ yards, they tested all students' scopes for tracking accuracy. I saw the pictures of where Frank stabilized the rifles to a bench rest with webbing ratchet tie-downs. Then they tested the scope tracking against tall paper at 100 yards, and calculated the scope tracking error rates for each rifle. They didn't fire the rifles for this tracking test. The students then built adjusted elevation tracking tables. I recall Frank's stating that the method worked quite well.

As usual, thanks for the comments, fritz. I believe I recall that you’ve mentioned before that tracking accuracy can fall off at the upper limits of a scope’s adjustments. I had forgotten that, but it’s definitely something to be aware of if the scope will be pushed to its mechanical limits. I’ve never seen it discussed by anyone else, but it certainly could be an issue.

quote:

Originally posted by joatmonv:
I call it a "Box Test"

I have never fired the box test, but if we’re referring to the same thing, and I’m pretty sure we are, it’s different from what I’m discussing.

The box test is a way of determining whether the scope tracks consistently up, down, right, left. That’s an important question because it can affect accuracy even at more typical shooting distances than the examples I’m discussing. The box test does not, however, tell us whether the adjustment values of the scope are correct, i.e., whether a milliradian-calibrated scope moves the reticle exactly 0.1 mil for every click or a minute of angle scope moves the reticle exactly 1/4 MOA for every click. The box test would put the last group on top of the first even if the adjustment values were not correct as long as they were all the same for the four directions of adjustment.

The potential problem I’m referring to if the adjustment values aren’t correct is if we use something like a ballistic calculator or dope table to adjust the scope for targets at different distances. Let’s say that the calculator says we need to move our point of impact up 50 inches at 650 yards, that will come out as a needed adjustment of our scope of 2.1 mils, or twenty-one 0.1 mil clicks. But if the scope’s adjustments move the elevation by only 0.09 mil per click despite claiming that they’re 0.1 mil clicks, then 21 clicks will move the point of aim up only about 1.9 mils, or 44 inches, and that could cause a miss.

A 6-inch discrepancy at 650 yards might not matter, but it really becomes important at much longer ranges such as the ones fritz cited.

The only way the box test would answer the question of whether the scope’s adjustment values are correct would be to measure the size of the box and compare it to what it should be at the shooting distance. But using fired groups would be subject to the same limits that I discussed above, especially if the box was fairly small.

quote:

Originally posted by sigfreund:

The potential problem I’m referring to if the adjustment values aren’t correct is if we use something like a ballistic calculator or dope table to adjust the scope for targets at different distances. Let’s say that the calculator says we need to move our point of impact up 50 inches at 650 yards, that will come out as a needed adjustment of our scope of 2.1 mils, or twenty-one 0.1 mil clicks. But if the scope’s adjustments move the elevation by only 0.09 mil per click despite claiming that they’re 0.1 mil clicks, then 21 clicks will move the point of aim up only about 1.9 mils, or 44 inches, and that could cause a miss.

A 6-inch discrepancy at 650 yards might not matter, but it really becomes important at much longer ranges such as the ones fritz cited.

I get it now and something that I should try to make sure that the scope is indeed making "X"mils adjustment per click.

The guys reviewing scopes use a rail section mounted to a base that they can level and mount on a table or tripod.
They use them for checking adjustments and if a particular mount will hold zero.

For an AR, just the upper in my tripod works for checking adjustments and bore sighting.
Don't think it would hold steady enough messing with the mount though.

quote:

Originally posted by powermad:
The guys reviewing scopes use a rail section mounted to a base that they can level and mount on a table or tripod.

I have a rail section attached to a board and with a quick detach plate on the bottom so the setup can be mounted on a tripod, but when I decided to check my scopes I didn’t want to remove them from the rifles and then have to go through the process of checking zeroes again.

That is a good idea, though.