Effects of Canting Rifles and Bullet Impact Changes

Discussion in 'ELR, Ballistics & Bullets Board' started by Bart B., Mar 21, 2017.

  1. Bart B.

    Bart B.

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    Noticed the following page:

    http://www.accurateshooter.com/optics/canting-effect-on-point-of-impact/

    With due respect its creator, it's bad information as the bullet's drop from maximum ordinate is used.

    Canting the rifle causes bullet impact to shift to the side equal to bullet drop at target range multiplied by the sine of the cant angle. It'll be below the horizontal equal to bullet drop at target range multiplied by the cosine of the cant angle, then that subtracted from bullet drop.

    Bullets drop the same distance from the line of fire above the aiming point regardless of how the rifle is canted.

    clock face.png

    Maximum ordinate (MO, high point of trajectory above LOS) for zero cant is almost 30 inches at 350 yards for a 600 yard zero for the bullet used to calculate data for.

    Note the drop below horizontal below the red line for different degrees of cant. As the sine of 30° is .5, note the 30° cant moves the bullet impact half of 100 inches; it's 50 inches to the right and several inches low. And those 5 bullet holes' plotted arc is the same arc as the clock face has at its edge.
     
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  2. mikecr

    mikecr

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    Important also is that folks understand that there is nothing wrong with a canted rifle -provided it's setup to do so with a plumb POA. This, including the rest.
    I shot a T2k at 7deg cant for years.
     
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  3. Keith Glasscock

    Keith Glasscock True believer - Straight 284 Gold $$ Contributor

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    I love seeing my competition in f-open stop shooting because the rifle is starting to cant. Nearly every time, they start losing points.

    I wonder what causes that?

    Seriously, how many out there have actually measured how many degrees of cant it takes to move the bubble in their scope level?
     
  4. Bart B.

    Bart B.

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    I've canted my front sight at angles needed to move bullet impact 1 and 2 MOA horizontally for different ranges, then noted where the bubble was. Works great in team matches when the coach gives a half MOA right command then I cant the rifle moving the bubble to the side some amount.

    sight bubble.jpg

    Mounted the sight on a yard stick, got it level, then put a half inch spacer at the 28-5/8" mark which tilted the sight 1 degree, then marked the sight. Then used Sierra's software to calculate bullet drop at range to calculate cant angles for a 1 and 2 MOA correction. Much easier that going out of position to twist the windage knob.

    Some smallbore prone folks have mounted their rear sight with its windage arm angled from 8 to 2 o'clock. Holding the rifle level, that compensated for cross winds making bullets go from 10 to 4 o'clock.
     
    Last edited: Mar 22, 2017
  5. Eternal Student

    Eternal Student

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    Bart- Not to sound like a bigger idiot than I am but you are saying that canting will cause you to shoot low to what ever side you are canted to IF the elevation is correct?? Because your rifle can not use the maximum elevation potential unless it is straight up. More or less.... Right?
     
  6. Nvreloader

    Nvreloader

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  7. 338 Mollett

    338 Mollett

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    Are we shooting sling now Bart?
     
  8. smoooth

    smoooth Gold $$ Contributor

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    I'm color blind, the graph confuses me?
     
  9. brians356

    brians356 Gold $$ Contributor

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    Don,

    Good one, thanks. Now I have to go check all my scopes. But it's not easy to verify the reticle and turrets are in alignment. I can get fairly good results if I can balance a good spirit level atop the elevation adjustment turret. (That assumes of course that the end of the turret is cut square to the axis of adjustment.)
    -
     
  10. Nvreloader

    Nvreloader

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    Brian
    I now use B Litz's Tall target test, on all my SP pistols/rifles etc.
    http://bulletin.accurateshooter.com/2015/11/calibrate-your-clicks-with-tall-target-test/

    Before tightening the scope down for the final time,
    this test has sure helped me at long range shooting,
    and I can check the scope adjustments, at the same time,
    to make sure they are as quoted by the scope maker.

    If they are not as quoted, I know this info from the tall target test,
    and can then add/note the difference in my ballistic application,
    I only run MOA scopes, at this time, being a hunter,
    and just starting into the target shooting end.

    I use 10 MOA adjustments per step up, when I test,
    then I can see any difference at each of the 10 MOA distances etc,
    I have been surprised a couple of times at the test results,
    as compare to OEM spec's.

    I also run a scope level,
    after the Tall target test results are perfected etc.

    Well worth the effort/time IMHO.

    HTH,

    Tia,
    Don
     
  11. brians356

    brians356 Gold $$ Contributor

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    Don,

    Thanks, I'd seen that before, will revisit again now. Of course, if the rifle only manages 2-inch groups on that target, it's not very useful, the smaller the groups the more reliable the resulting calibration.
    -
     
  12. Nvreloader

    Nvreloader

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    Brian
    I hear what you are saying,
    I have numerous groups right at or just under 1",
    with my short 21" bbl SP Pistol, standard 284,
    @ 300yds, 140gr Nosler Part bullets,
    which I am tickled with.
    Groups were running 2-1/2 to 3" before the tall test.

    Tia,
    Don
     
  13. mr45man

    mr45man Gold $$ Contributor

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    Im not but where is the RED line?
     
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  14. 6MMsteve

    6MMsteve Gold $$ Contributor

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    bottom of the graphics
     
  15. 6MMsteve

    6MMsteve Gold $$ Contributor

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    can this actually help when shooting in strong gusty winds, canting the rifle to drive the bullet to a path
     
  16. JPeelen

    JPeelen

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    In principle, you could compensate crosswind by deliberately canting, mostly because cant primarily produces a sideways deviation compared to a very small vertical deviation (see graphic in the first message in this thread).
    But determining the correct cant angle is, in my estimation, about 10 times more complicated than simply holding off.
    In strong crosswinds, this would become about 50 times more complicated than simply holding off, because you also need to compensate for the bullet drop caused by cant.
    Did you ever hear of any experienced shooter (Thompson-Gallagher, Tubb, you name them) adopting this crazy scheme?
     
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  17. brians356

    brians356 Gold $$ Contributor

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    No, but I've heard of Bart B. doing it.
    -
     
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  18. Bart B.

    Bart B.

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    John Unerl (target scope maker) suggested I tack a yardstick to something 100 yards away so as to be seen in a rifle scope clamped solid in place aligned on it near one end.

    Note where the crosshair intersected a measurement, then move the adjustment 20 MOA. Note where the reticle stopped.

    19.2 inch/MOA movement? 20.2 inch/MOA movement"?

    Do the grade school math to see exactly how much 1, 4, or whatever.... clicks moved bullet impact. With a 20X scope, your error will probably be less than 1/16th inch/MOA. You'll not even be close boxing or stringing shots that has that small error level. Unless you shoot groups half that size all the time.

    All internally adjusted scopes of the same make and model will not have exactly the same movement per click. A 1 to 2 percent spread is normal. Lenses cannot be made to the same optical tolerances as mechanical parts to dimensional tolerances.

    If you don't like the USA shooting sport's MOA standard of 1 inch per 100 yards, put the yardstick 95.4936544467 yards away. Then an inch on the yardstick will be the cumbersome, but very precise, trigonometric MOA value simplified for you.
     
    Last edited: May 5, 2018
  19. brians356

    brians356 Gold $$ Contributor

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    Good approach, but that needs to be a very solid scope clamp setup.
    -
     
  20. Bart B.

    Bart B.

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    Yes indeed.

    Once set up, you can change a zoom scope power to see if the reticle stays fixed on target. Many zoom scopes have enough slop in zoom lenses' fit that the reticle makes a figure S or 8 pattern changing back and forth from low to high power then back to low.
     

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