wider groupings with higher magnification @100???

[Crosshairs6]

As a former USMC Sniper Instructor, I have seem this many times in "grouping drills" at 100 yards. Assuming your rifle is put together correctly, and from what you are stating it is, it seems to me that as you are squeezing the trigger, you are watching the target and not the crosshairs. You body postion keeps you on target. You must have crystal clear crosshairs when the shot breaks. If not, your groups will be huge.

 

[gstaylorg]

There are potentially several reasons to explain the inconsistent group sizes when changing hi/lo magnification at 100 yd, as has been mentioned in the replies above. One point that needs clarification, however, is with regard to parallax. Parallax error increases with magnification as was stated, but it actually increases as distance decreases. The closer your target, the more critical it is to make sure your parallax is correctly adjusted. At 100 yd, parallax error can be pretty dramatic and will only be further amplified with higher magnification.

If the error is due to a tendency to follow the previous shot hole with the crosshair, one trick to get away from that is to purposely set your elevation half to one MOA high/low. That way, you're not aiming directly at your POI or shooting out your aiming point. It's much easier to hold steady and pay attention to what your crosshair is doing as you shoot when your POA isn't shot up or surrounded by a bunch of holes in the target.

 

[Sterling Shooter]

There are potentially several reasons to explain the inconsistent group sizes when changing hi/lo magnification at 100 yd, as has been mentioned in the replies above. One point that needs clarification, however, is with regard to parallax. Parallax error increases with magnification as was stated, but it actually increases as distance decreases. The closer your target, the more critical it is to make sure your parallax is correctly adjusted. At 100 yd, parallax error can be pretty dramatic and will only be further amplified with higher magnification.

If the error is due to a tendency to follow the previous shot hole with the crosshair, one trick to get away from that is to purposely set your elevation half to one MOA high/low. That way, you're not aiming directly at your POI or shooting out your aiming point. It's much easier to hold steady and pay attention to what your crosshair is doing as you shoot when your POA isn't shot up or surrounded by a bunch of holes in the target.

Can you explain with a few more details about parallax decreasing with distance. Your statement makes me wonder about it since parallax is a perspective of aim error (sight misalignment error) which is angular and being angular it will increase with distance.

 

[gstaylorg]

If you think about the underlying cause of parallax error, which is basically a transient misalignment between shooter/lens/reticle/lens/target, the angles become greater the closer the target. The misalignment angles for targets that are very far off are become smaller and therefore the angular error introduced is less. For that reason, past a certain distance, the parallax setting on most scopes is "infinity". I should point out that what I referred to as "parallax error" in the previous post is not exactly true from a mathematical sense. The angular error changes with respect to distance, but that is only part of the story as what you really want to know is the linear distance a shot will be displaced by such error, which can be determined from the angular value and the target distance. I realize this isn't the best explanation, if you're interested, the following link has a much more thorough explanation:

Worth Reading

What you can see from the calculations in this article is that parallax error in terms of MOA is generally much higher at close range. Although it can start to increase again from a minimum value as the distance goes up, it will not normally reach nearly the maximum amount observed at close range. Calculating the maximum linear distance this will move the POA, it can be seen that it decreases with distance.

For me, the simplest demonstration of this principle is to move my head side to side while looking through the scope with the parallax setting purposely off. Although you have to take increased distance and angular movement into account for the farther target, it seems pretty clear that the amount the reticle seems to "move" across the target face is much greater close in. Parallax error has been a known issue for smallbore shooters for many years due to the shorter distances they normally shoot.

 

[Sterling Shooter]

So the angle is greater at shorter distances not the value which is a constant for whatever the angle; therefore, the value at increased distance still increases the amount of displacement at distance no matter what the angle.

 

[Djstorm100]

How do you check to see if you parallax is correct? I have a cheap muelluer scope on my .22 trainer that I've been using as a learning tool before getting in to a heavier cost caliber.

 

[ahage2]

How do you check to see if you parallax is correct? I have a cheap muelluer scope on my .22 trainer that I've been using as a learning tool before getting in to a heavier cost caliber.

Definitely not a pro here, but what I've heard a handful of times is to wiggle your head side to side and/or up and down while looking through the scope...if the crosshairs jump around, it is set wrong...

Adam

 

[gstaylorg]

Perhaps I didn't state it very clearly. I understand the concept, but it's a difficult one to explain and it's not linear. It's probably easier if you look at the displacement values from the article linked above. You can see that they decrease with respect to maximum calculated parallax error as distance increases, reaching a minimum at 150 yd or so in this example, then go up slightly:


Maximum PE @ 25 yards = 0.5 * 32* ABS(25-100)/100 = 12mm or 0.47 inches
Maximum PE @ 50 yards = 0.5 * 32* ABS(50-100)/100 = 8mm or 0.32 inches
Maximum PE @ 150 yards = 0.5 * 32* ABS(150-100)/100 = 8mm or 0.32 inches
Maximum PE @ 175 yards = 0.5 * 32* ABS(175-100)/100 = 12mm or 0.47 inches
Maximum PE @ 200 yards = 0.5 * 32* ABS(200-100)/100 = 16mm or 0.63 inches
Maximum PE @ 300 yards = 0.5 * 32* ABS(300-100)/100 = 32mm or 1.26 inches



Maximum PE @25 yards in MOA = 12mm *3438/ 22860mm = 1.80 MOA
Maximum PE @50 yards in MOA = 8mm *3438/ 45720mm = 0.90 MOA
Maximum PE @150 yards in MOA = 8mm *3438/ 137160mm = 0.20 MOA
Maximum PE @175 yards in MOA = 12mm *3438/ 160020mm = 0.26 MOA
Maximum PE @200 yards in MOA = 16mm *3438/ 182880mm = 0.30 MOA
Maximum PE [MENTION=89035]300[/MENTION] yards in MOA = 32mm *3438/ 274320mm = 0.40 MOA

If you plug in values for 1000 yd, you will get 144 mm/5.67 inches/0.54 MOA.

So in effect, what I stated is only true within distances from very close out to somewhere between 150 and 200 yd (25 yd to ~150/175 yd in the above examples). After that distance, the parallax error does increase slightly as distance increases, although I think it's a small enough increase that it gets lost in the noise, so to speak. In the above example where maximum parallax error goes from 0.40 MOA at 300 yd to 0.54 MOA at 1000 yd, I think it's safe to say that for most shooters, other factors including wind, external ballistics, and shooter error will increase their average group spread by a lot more than 0.14 MOA when going from 300 yd to 1000 yd. Obviously the exact values for PE will change for scopes with different exit pupil, magnification, and objective lens diameter, although the relative trend (shape of the curve) should remain similar.

It can be argued from the above data that my original statement regarding parallax error decreasing as distance increases is only true within a specified range of distances, even though I believe the gradual slight increase in parallax error becomes less significant past 300-500 yd as other factors begin to influence group dispersion to a larger degree. In the context of the original post, which was with regard to the effect of increasing magnification on group dispersion, it's clear that parallax error increases in directly proportion to the increase in magnification. For that reason, parallax error is one possible reason (among many others) for groups to open up at 100 yd as the scope mag was changed from 9X to 20X.

 

[Sterling Shooter]

Not too long ago, while teaching Soldiers how to use their ACOG's I noted that an eyeball not squared with the eyepiece could result in about 18 inches of error at 600 meters. That's a lot of error, and not easily confused from error produced from poor position or trigger control. On the other hand, I did have one knucklehead who thought his error was all about his misreading of wind when the reality was he did not maintain a consistent stockweld from shot to shot. At any rate, I see that you do agree that the effect of parallax at target does indeed increase with distance as does any sight misalignment error.