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  • Check your scopes!!!

    I knew it was a good idea to check the comeups on scopes, especially ones that rely on BDCs. Now I believe it's essential.

    If you're going to be shooting small targets across the course, then you have to know where you're going to hit. A big part of that is keeping a data book. But, the learning curve can be shortened considerably by checking the expected comeups right out of the gate. Knowing where your rifle actually WILL shoot will help answer those nagging questions, like why am I only needing to dial 400 yards for that 425 yard target, when I should be having to dial 400+1MOA? Ah! Mystery solved.

    This is my test of my U.S.Optics MST-100.

    I guess this is what is called a "tall target test"?
    Any way, the orange circle is the single aiming point throughout the test. I believe I dialed an additional +1 or 1 1/2 MOA for the entire test to keep the aiming mark fresh.
    The target was set at a measured 286.5 feet, so that one inch equaled one Minute-of-Angle to simplify the necessary post-shooting calculations.
    The vertical sighting line was plumbed.
    The two bold vertical lines ( l l ) were to help keep the vertical crosshair right on the line. Lining up the orange aiming mark and the bold centering marks ensured shot-to-shot consistency.

    The course of fire was round robin to eliminate any effects of scope adjustment backlash. One shot at each scope turret setting from 100-1000, then from 1000 back down to 100, back up, etc. for a total of five shots at each range setting to help find the average group elevation at each setting.

    Now, there were NO NOTATIONS on this target while it was being shot. In other words, and this is important, I wanted to know where my rifle was shooting, not where I was expecting it to shoot.
    The bold yardage markings on the target are the actual centers of the groups for each turret marking in hundreds of yards.

    The numbers on the left margin in boxes are the actual elevation POI in MOA for each hundred yard setting adjusted for a 300 yard zero. In other words, those numbers represent the comeups assuming that the 300 yard group is zero (at the factory specified 5 MOA @ 300 yards).

    The numbers between the boxed numbers and the bold yardage lines are the actual elevation POI in MOA for each hundred yard setting adjusted for a 100 yard zero.

    I'll follow the photos with the analysis, and what I learned about my scope's actual comeups (they weren't all at the expected advertised comeups!).

    The target...





    The "key"...











    The first thing I noticed was that the scope seemed to track really well as far as internal backlash went, with all the groups holding under .75 MOA vertical spread (.48 MOA average) in spite of the round robin format. Taking into account normal group dispersion, this is extremely good.

    The second thing I noticed was that the scope did not track perfectly on the vertical. The 100 yard group was right of the sighting line by .26 MOA, to .67 MOA for the 1000 yard group, with the intermediate groups more or less holding the same line. That makes a total of .41 MOA windage shift from 100 yards to 1000 yards. I suppose that .41 MOA at 1000 yards would become lost in the wind, so maybe that's not too bad.

    Otherwise, the scope's range settings matched the published comeups pretty well. But the settings at 100, 400, and 700 yards were "off" enough to warrant making corrections when using them.
    These "off" settings explain why I was not able to get my scope to track perfectly from 100 yards to 1000 yards, since for the usual 300 yard zero my actual 100 yard POI was .75 MOA HIGH!
    The "off" 400 yard setting also explained why I always only needed 400 yards on the scope to hit a 425 yard target on center (should have needed 400+1MOA @ 425 yards). Enough to miss clay pigeons.

    Here are how the pubished comeups compared to my actual comeups determined by POI at each scope setting on the tall target. I don't know how to make this in chart form, so I'll run them left to right at the 100 yard intervals so they can be compared vertically (my settings rounded to the nearest 1/2 MOA)...
    USO published scope comeups: --------0---2-----5---8-----12------16-----20.5---25-----31---37.5
    Adjusted for a 100 yard zero (0 MOA): 0---1.5--4---7.5---11.5---15.5---20-----24.5---30---36.5
    Adjusted for a 300 yard zero (5 MOA): 1---2-----5---8.5---12------16-----21-----25-----31---37.5

    As can be seen, a 300 yard zero lines up the actual comeups with the published comeups much better than a 100 yard zero would on this particular scope. I just have to remember to dial down 1 MOA at 100 yards, and down .5 MOA at the 400 and 700 yard settings.

    This information is also invaluable when tuning loads to track the scope's BDC. If I based the trajectory on data for 100 yards and 500 yards, I'd be "off" because my 100 yard mark is "off" by .75 MOA as determined by the test. I have to know to account for that if I want to use a 100 yard target for tuning.
    Last edited by SemperFi; 09-25-2017, 10:38 PM.
    You can take a Marine out of the Corps, but you can't take the Corps out of a Marine.

  • #2
    Nice work.

    Did you adjust the parallax out at the distance you were shooting?

    Comment


    • SemperFi
      SemperFi commented
      Editing a comment
      I kept the parallax at the 300 yard setting throughout the test.
      Last edited by SemperFi; 09-27-2017, 02:05 PM.

    • SemperFi
      SemperFi commented
      Editing a comment
      I edited the last part of my previous comment because it was nonsense. I had the parallax set at 300 yards and kept it there for the test. In fact, that was the main reason I had to make the bold reference lines ( l l ) to help align the cross hair, as I was unable to see the aiming line with the parallax set the way it was. It would have been better to set the parallax for target distance (the groups would have been a little smaller), but you know how setting parallax is on these, and I was being lazy/saving ammunition.

  • #3
    Was your MV, load, bullet weight, barrel length and twist rate, and atmospheric conditions all identical to the design data and parameters that were used by the manufacturer for BDC values? Even if you did replicate all these as close as possible, manufacturing variances between individual rifles and ammo will show discrepancies to nominated BDC values. If you did not match these then all you have achieved is to test / verify your come ups for your particular load (dope), which we should all be doing every time we load a new batch or buy a new batch of ammo. Nice test BTW.

    Comment


    • SemperFi
      SemperFi commented
      Editing a comment
      No, not for this test. As long as the bullets and MVs are consistently the same shot-to-shot, the actual trajectory is irrelevant since the distance does not change.

    • brent65
      brent65 commented
      Editing a comment
      Ok I'm out you have completely lost me now, lol. Maybe it is just my simple mind or I am over complicating it but I thought bullet gr, bc and MV would have everything to do with replicating the BDC values, otherwise to me it just seems like a tracking test. Granted at 100yds the variation would be minimal but 100FPS variation from design MV would show on the test as would using 155's in lieu of 175's for example.

    • SemperFi
      SemperFi commented
      Editing a comment
      It's true that changing MV by 100 fps or changing bullets dramatically would likely give a different POI at a given distance, but that difference in POI would be the same for every shot. What is being tested, basically, is where the marks are placed on the turret. 1 MOA is 1 MOA. If the erector in the scope body is mechanically moved to shift the line of sight through the scope by an angle of 1 MOA, the POI, wherever it may have been, will likewise shift 1 MOA.

      In other words, imagine you have one load that prints 1" lower than another load, and you perform this same test side by side with each load independently (assuming the environmental conditions don't change significantly during the tests). The groups in the test using the lower printing load will all be 1" lower throughout the test in relation to the corresponding groups in the test using the higher printing load, but the spaces between groups will be exactly the same for each corresponding change in scope settings.
      Last edited by SemperFi; 09-27-2017, 01:36 AM.

  • #4
    Very nice piece of work!

    Comment


    • SemperFi
      SemperFi commented
      Editing a comment
      Thank you.

  • #5
    This same test can be performed eliminating any variation due to ammunition by using a laser and a buddy down range to mark your target while you sight in at each elevation setting. 286.5 feet is the magic distance to make inches equal MOA.
    You can take a Marine out of the Corps, but you can't take the Corps out of a Marine.

    Comment


    • brent65
      brent65 commented
      Editing a comment
      I do like the test BTW

  • #6
    Regarding the vertical alignment 'drift': are we certain the scope is absolutely perpendicular? I could see either/or/combination of scope rotation in rings; cant of rifle/scope causing this drift...

    Imagine scope is rotated slightly counter-clockwise from "plumb" with bore in the rings and then the rifle is canted right slightly to make crosshair vertical...

    Just throwing some over-active engineer in the mix perhaps, but it is certainly possible moving something within a thousandth of an inch nearly 300 feet away there could be error... Let's bear in mind exactly what we are asking of these ancient designs!

    Before I threw out the scope with the bath water I'd want a precision test stand with no horizontal movement (just a hinge for vertical), a laser pointer and a thumb screw to elevate the scope and align the cross hairs after elevation adjustments are made. Everything level and plumb to the best of optical ability. THEN we know what the scope is doing.

    Not that this test does not have value- on the contrary, all dope is only good for the shooter operating the system with their ways of doing things. But to get "laboratory standard" adjustments, I think a "silly rig" is required...

    Comment


    • J!m
      J!m commented
      Editing a comment
      I may make a "silly rig" for fun... It would be fast and easy and I could check any scope for anyone. And it's quiet! No need to fire a round to generate "mechanical dope" for any given scope.

      I used to invent stuff in a prior job and this thread got me thinking again.

    • SemperFi
      SemperFi commented
      Editing a comment
      I realize this may sound counter intuitive, but it doesn't matter if the scope is mounted with the vertical cross hair perfectly perpendicular to the center of the bore or not for this test. You could even have the scope mounted at 90 degrees from the center of the bore and the results of the test would be the same. Again, this is because the test is only performed at one distance. Canted scope mounting only affects POI when the distance is lengthened or shortened. What matters for this single distance test is that the aiming line is plumb, and the vertical cross hair remains superimposed on it (plumb) when the shot is fired.

      I'll even go out on a limb here and say that the aiming line doesn't even have to be plumb. As long as the distance remains constant and the vertical cross hair is superimposed consistently on the aiming line, the POI will always track the internal shift of the erector tube as the elevation knob is turned. But, to simplify the discussion, let's assume the aiming line is plumb.
      Last edited by SemperFi; 09-27-2017, 10:28 AM.

    • J!m
      J!m commented
      Editing a comment
      I believe you are right; however if the scope (for whatever reason) induces 'cant' of any kind when aligned/fired, and that degree of cant is not held constant, it induces an error.

      But if the vertical travel is held such that no additional horizontal movement can happen, then you are absolutely right that the scope could even be mounted four feet from the bore and still have good results.

      Who wants to deck screw a door hinge to the butt of their M40?
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