OK, I am new the the whole precision rifle aspect. A DMR class is in my plans. I do have the marksmanship fundamentals down (natural aiming position, breathing, trigger squeeze and hold).
I recently completed a Mk12 variation with a Centurion Mk12 barrel, Vltor upper, BCM BCG, Surefire Brake, PSA lower, Magpul PRS, Geissele SSA-E, No name 2.5-10x42 scope (waiting for my PSL to sell so I can get the NF 2.5-10), on a Burris PEBR mount.
I ran this rifle through a 2 day carbine class last weekend and TD1 and half way through TD2 my zero was still dead on. In the afternoon the wind kicked up to about 10mph, with gust to 15, full head wind.
Doing a 100y 10 round, prone off the mag, as far as I can tell my position was locked in. After every shot the cross landed right back where I pulled the trigger, and every shot taken at the same point of aim. My group was tight with a slight left/right drift, but 2" high. I checked to make sure that nothing had come loose or any obvious movement/drifting.
I would have thought a headwind would have caused a drop in POI.
There are some factors that may have made a change, but I can’t account for 2". These are my M193 reloads, LC brass trimmed at 1.749, Hornady 55gr FMJBT, AA2230 with +/- .01gr variation, COAL 2.20 +/- .002. I can get consistent 1.5 MOA 10 round groups with this ammo in my Centurion 16" middy.
Scope did not get smacked, and the turrets were still where they were set when I zeroed.
Wind, shooter error (entirely possible I did not have the same cheek weld when I zeroed, was not thinking of that at the time), or Scope?
Had the same issue again when we ran the VTAC on a full IPSC steel @100 with a good 20-30* down angle. Aiming at the head/shoulder cross, and completely missed over the top. By this time the wind was a consistent 15mph and 5* off to the right.
I will try to get out in the next week and verify that I still have zero at the same indoor range I started at.
I can’t diagnose your issue from here, but it sounds either optic (you mentioned it was a no-name brand) or shooter-position related. Going from resting the forearm on an obstacle to a magazine monopod, for example, can move POI around a bit. You also mentioned that the missed IPSC steel target was at a 20-30 degree down angle – which means that you have to aim lower (you might already know this).
Anyway, just for general information’s sake, I’ll repost two previous responses I’ve given on these subjects.
The first section will be on “No Value” winds. Keep in mind that this is more of an academic discussion and has little bearing on the real world unless you’re shoooting benchrest competition where a centimeter or two matters.
The second section will be on shooting at up or down angles.
Section 1:
Winds from the 6 and the 12 are commonly called no value winds.
While head and tail winds are said to have no value, in reality they do have a very minor effect.
Conventional wisdom says shooting into a headwind lowers POI. Shooting with a tailwind raises it.
It’s a function of time of flight more than anything else. Remember, two identical objects will fall at the same rate regardless of velocity. A loose bullet simply dropped by hand will hit the ground at the exact same time as a bullet fired at 3,000 FPS on a level plane from the same height.
Since a headwind increases TOF your POI will go down because gravity will have more time to “pull the bullet down.”
How much? A 77 GR SMK out of a MK12, for example, will need ~ 0.3 more MILS of UP elevation adjustment at 1000m with a 20 MPH headwind. (I’m using extreme numbers to illustrate that while there IS a difference, it is very slight.) Use a realistic range and 10MPH wind and you’ll find that you don’t need to worry about it much.
^^^ That’s conventional wisdom.
I have my own thoughts. Here they are:
This diagram is something I’ve come up with that IMO is a bit more accurate in certain circumstances.
When you have a Zero where the trajectory of the round crosses above LOS, an increase in the bullet’s TOF on the way to the target (but on the near side of the no wind apogee…which also coincides with maximum ordinate of the trajectory), can actually require HOLDUNDER.
Why? Because until the round hits max-ord, the round is still moving up relative to LOS. Therefore, increasing TOF to a short range target (any range before max-ord) can and does make for a higher, not lower POI. Once the round hits the highest point of the trajectory, revert back to the conventional answer I outlined above and expect to see a lower POI.
All of this is pretty academic though. Unless you’re a competition BR shooter none of it matters much IMO. I don’t believe headwind or tailwind will sufficiently raise or lower POI to any quantifiable or significant level in a tactical environment.
Also, here is a better set of rules than thinking using the traditional wind values method, which is actually pretty wrong.
MPH X C = adjusted MPH
If wind direction is 90 degrees from sightline C = 1.0
If wind direction is 65 degrees from sightline C = .9
If wind direction is 45 degrees from sightline C = .75
If wind direction is 30 degrees from sightline C = .5
If wind direction is 15 degrees from sightline C = .25
If wind direction is 0 degrees from sightline C = 0
Section 2: Shooting at an angle.
In this post, I respond to the following question, originally posed on another forum: “Would you please explain incline / decline shooting in a paragraph or two?”
Gravity obviously has a different effect on trajectory when shooting on a flat, level plane than it does when shooting up or down. When an angle between the shooter and the target is introduced, either up OR down, gravity will have less of an effect.
There are a few ways to account for this.
The first is called “The Rifleman’s Rule.” It uses the shot angle multiplied by the measured range to come up with an “adjusted range.”
Range X C = adjusted range
5 degrees from horizontal C = .99
10 degrees from horizontal C = .98
15 degrees from horizontal C = .96
20 degrees from horizontal C = .94
25 degrees from horizontal C = .91
30 degrees from horizontal C = .87
35 degrees from horizontal C = .82
40 degrees from horizontal C = .77
45 degrees from horizontal C = .70
50 degrees from horizontal C = .64
55 degrees from horizontal C = .57
60 degrees from horizontal C = .50
65 degrees from horizontal C = .42
70 degrees from horizontal C = .34
75 degrees from horizontal C = .26
80 degrees from horizontal C = .17
85 degrees from horizontal C = .09
90 degrees from horizontal C = 0
So, if your target is 500M away, but at a 60 degree angle (up OR down), you could using this first method, multiply 500 by .5 and come up with an adjusted range of 250M. You would then treat the target as if it was 250M away and adjust accordingly.
The next way to solve the problem is to simply apply the cosine to the adjustment instead of to the range. This is called the Improved Rifleman’s Rule. So if you need 5 MOA of adjustment to make a 500M shot with no up or down angle…but then use the scenario above where you have a 500M distance with a 50 degree angle, you would simply multiply 5 MOA x .5 and use 2.5 MOA of adjustment.
The two methods will usually come close, but DO produce different results – and the second way is generally preferred.
The problem is that neither of these methods are all that accurate, especially as the angles become more extreme.
Good ballistics software is able to account for an incredible amount of variables, including environmentals and the ballistic performance of individual rounds. The best solution is to plug the data into a PDA for a guaranteed first round hit.
For general purposes, just keep in mind that you will have to aim lower for a given distance if there is inclination or declination involved.
Here’s a graphic representation of what’s going on:
Thank you. Answered a lot and more (plans on taking a DMR class soon)
I should have posted the ammo - Home baked M193 - ETA I did…really needed to re-read my post.
I am a mechanic by trade, so when something goes wrong all of a sudden I can’t let it go until I figure it out, and the diagnosing begins.
TD1 went fine aside from having to mortar a few rounds, but my zero was still dead on at the end of the day.
TD2 was a less aggressive beginning, and thinking through everything, the rifle never got bumped, smacked, mortared, or any other shock than being fired.
Even if the zero had shifted on a POS no name scope, I would not have expected that much at a mere 100y, or even much change in POI from wind at 100, but I have seen stranger things happen that I would not have believed if I had not been there.
New rifle, and first time running a rifle with a scope for that much length of time, let alone in a mid level carbine operator course. Might very well be shooter error in cheek weld, eye relief and more. Just annoying that the hits were that far off as solid as my prone position was (textbook insofar as natural point of aim, and minimal recoil movement through the optic).
I regularly shoot out to 600 yards in NRA high power with a 20" .223 rifle. I have never needed to factor in the velocity of a zero value, i.e. head or tailwind into my firing solution.
