Ballistics for shooting sideways; rifle canted at 90 degrees

psycowilley

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Apr 22, 2018
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I am getting into PRS shooting. Looking at all of the shooting scenarios makes me want drink more whiskey; that is what them dudes setting up the ranges must have been doing. One of the positions I see that can be quite complex, is the shooting sideways; rifle canted at 90 degrees; let's say to the left. Now your windage turret is on top and works as elevation, and your elevation turret is on the left and works as your windage. Is there anything out there that can help us out with the ballistics of this? I have been running this through my head the past few days and have some ideas. Picture the bore axis vs line of sight. When you shoot, the bullet goes up past you LOS and comes back down to your LOS at you zero distance. Now when you turn it all sideways, the travel of the bullet doesn't go in the arc any more looking from the top. (which used to be the side). Now you see an 'X' where your LOS and bore axis cross. So the bullet is now crossing your LOS (30-50 yards) and traveling away from your LOS; kind of like the diagram of "if there was no gravity, this is what your bullet would do." On top of that, it is drastically arcing down away from your bore axis because they are on the same horizontal line (instead of pointing up to cross your LOS and come down). If your LOS crosses your bore axis at 33 yards, and your scope heights is 2", then that would put your bullet about 2" left at 66 yards, and 4" left of point of aim at 99 yards; and about 5" to 10" low. However, the angle/offset doesn't double at 200 yards, because the angle started at the 33 yard line; the left/right angle will start at 33, go to 133, then double at 233. So, are there any ballistic charts or programs that help figure the ballistics of this? I'm betting that once we figure all of this out, someone will just make the targets farther away so we have to do more math. I bet there is some PRS range set-up dude reading this right now, drinking some Jack, thinking "damn, great idea! And they gona shoot it from the support side!"
 

mcameron

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  • Nov 17, 2011
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    stop thinking.....just shoot.

    try it at the range at some known distances......see how much it effects your grouping.....keep that approximate offset in mind, and apply Kentucky windage should you have to do it in a match.
     
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    LawnMM

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    The bullet crosses your line of sight AT the zero distance and comes back down beyond it. That's how a 50yd battle zero works on gas guns.

    The bullet leaves the barrel in the same way regardless of how the rifle is oriented. What's different is where the reticle is looking in the different orientations.

    Verify your zero, rotate 90 degrees, see where it lands and note the offset. Done.
     
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    lash

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    I <snip> I'm betting that once we figure all of this out, someone will just make the targets farther away so we have to do more math. I bet there is some PRS range set-up dude reading this right now, drinking some Jack, thinking "damn, great idea! And they gona shoot it from the support side!"
    Thanks for the tip. I'll change up the distances next time for you... ;):p:cool:
     
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    Greg Langelius *

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    The guys who have a need for wining are probably honing those skills at home so they can excel at the comp. The other guys come to the comp to seek the challenge and to learn it under the stress of competition. Each gets what they want.

    Greg
     

    TacticalDillhole

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    Nothing changes when you do this as far as what the bullet does. The only difference is you need to account for your offset. If your scope height over bore is normally 2.5” it’s now zero and the offset is 2.5” left. That’s it. Then like you said windage is elevation and elevation is windage.
     

    hereinaz

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    Elevation is also dialed in for the zero too, besides the offset, right?
     

    TacticalDillhole

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    Elevation is also dialed in for the zero too, besides the offset, right?
    Well yes, i assumed that would be a known since the windage knob doesnt have your zero. but what i was referring to is how you get the result from your BC to develop a range card. in order for it to work, you need to know how far up from the mechanical bottom your zero was and accomodate for that in the windage?it will involve holding more than dialing since a typical windage knob doesnt have nearly the travel an elevation one does. and this goes for wind also, forget about dialing, you will need to hold. this involves a lot of using the reticle.
     

    LH_Gina

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    I've never seen this as a course of fire past 100 yards, so getting dope for it should be relatively easy. While you're checking your dope with the port up, you might as well check it port down as well (because it's different). Haven't seen this drill used in a match in over 5 years so I'd guess you're safe from it popping up at a comp. From what I remember, most competitors hated it with a passion.
     
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    Dtarget

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    Just hold high and to the mag side. Knowing the approx offset helps.
     

    jitter

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    Will try to help...
    There are two major issues:
    1. zeroing after canting
    2. the rest ballistics from canting position.

    1) if a rifle is canted to the left (the riflescope is on the left), then your bore axis “looks” to thee left (to the riflescope side) and you need to move it back to the “bore side”, so you dial ELEVATION DOWN.
    In vertical plane now the rifle shoots with riflescope bore height(BH) =zero, so it shoots low and you need to get it up, so you dial WINDAGE RIGHT.
    (For my 260 with BH=5.8cm it’s like 1mil elev down (move POI right), 1.3 mil windage right (move POI up).
    2) After carrying out 1) we have our rifle zeroed in at 100.
    But! Due to a non-zero BH the bore “looks” to the left and crosses LOS, so throught the range POI will move to the left from LOS and we need to correct it (move to the right in some way).
    Another issue is that ballistics calculator gives the trajectory taking into account BH which now =zero.

    So as for BH=0 there are two ways:
    1. Set BH=0 and get new correct elevation
    2. correcting “old ballistics”(or dope) the same way as windage would be corrected.

    How do we correct windage with this BH offset?
    The additional windage (this is elevation knob which we turn down) is
    Wind_corr=BHa*(D100-1)/D100, where
    BHa - is a “angled” riflescope bore height that is if I have 5.8cm, then at the distance of zeroing it would be some angle which we calculate as 5.8/2.9 as for moa at 100m or 5.8/2.5 at 100y or 5.8/10 as for mil at 100m. I my case with miling reticle I get 0.58mil. This is the angle which bore axis has with LOS looking from the zeroing point.
    D100 – hundreds of meters/yards to the target.

    Let’s say we shoot at 300, so D100=3
    And the windadge correction for my data(BH=5.8cm = 0.58mil) is 0.58*(3-1)/3 = 0.58*2/3=0,39mil.
    So in addition to the zeroing “elevation down” shooting beyond that point I need to move POI “more down” at +0,39mil

    This was about windage correction (by elevation knob turning it down).
    The same way elevation is corrected. Additional elevation correction value
    Elev_corr=BHa*(D100-1)/D100
    This is needed because the bal.calc accounts nonzero BH which is now zero and we need to get trajectory up by the angle which we would have had if we had that BH nonzero.
    So at 300 it’s still plus 0.39mil from my example.

    So for shooting canted to 90 degree we have 3 components: zeroing shift + data from bal.calc(or dope for the BH non-zero) + Wind/Elev correction (or only wind correction if ballistics is calculated with BH=0).
     

    Clearlight

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    As above , but just as important you have to actually practise shooting from 90 left and right cant .
    Not ideal to drop your windage or el turret on the obstacle , so you need to practise setting up
    whatever bags you run to make the shot . A braked 6mm is pretty easy , unbraked 30 cal less so .
    If running a brake , you may need to be aware of what your brake ports are aiming at too .
     

    RocketManStan

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    Jan 1, 2014
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    Wow. A lot of information out there about POA/POI when rotating a rifle 90 degrees. From what I can see, a lot of it either assumes the target is at the distance your optic is zeroed, or the advice is essentially to “guess.” Here’s how to calculate EXACTLY the POA needed, regardless of the distance to the target and the distance you chose to zero your optic:

    First, the drop. With the rifle at 90 degrees, the optic is vertically at the same elevation as the barrel, but the bullet begins dropping the instant it exits. So use your ballistics program, leave all parameters as are for you current ammo/rifle, but set the sight height to .01 inch and the zero range to 10 yards. Bingo! The resultant trajectory will give you the drop at any distance. That is, the amount of vertical “holdover” you will need!

    Second, the horizontal offset. If your sight height is, say, 3.00 inches, your POI will be 3.00 inches to the “inside” (towards the magazine) from the POA when the rifle is held at 90 degrees. As there is no gravity working horizontally, the POI will continue to move towards the POA at a constant rate with distance, namely at the rate determined by the angle between your line of sight and the barrel (which itself is determined by your ballistics and your chosen zero range). How to determine that rate? Look at a normal run of your ballistics program and subtract the trajectory number at 10 yards from 0 yards. (Yes, for you high school physics teachers, that ignores the drop in the first 10 yards due to gravity, but if you want to add that in, distance= (accel x time^2)/2, so for a 3000 fps projectile the added drop is 0.002 in. My eyes are not that good.)

    That 0-10 yard change in trajectory is the amount the bullet POI moves towards the “outside” every 10 yards…constantly throughout its flight. If your (3000 fps) rifle is zeroed at 200 yards, you’ll find the “crossover” is at about 55 yards. That is, the horizontal offset decreases to zero at ~55 yards, then the horizontal offset starts to increase to the other side beyond 55 yards.

    You can easily make a spread sheet to give the offset and drop, say, at 10 yard increments, for your particular setup. For example, for my setup (3067 fps 5.56 55gr B1 .254, 3.00 in optic height, and 200 yd zero), here's a few selected offsets/holdovers (positive offsets being “above” the desired POI from the rifle’s point of view, negative offsets being “below” from the rifle’s point of view):

    30 yd offset 1.26 in holdover 0.14 in

    100 yd offset -2.80 in holdover 1.90 in

    300 yd offset -14.40 in holdover 21.67 in

    Best to do your printouts for every 10 yards from zero to the max you’ll be shooting. Do your calculations, know your distances, and hit your targets! Simple stuff!
     
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    ivesh

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    Nov 13, 2020
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    Hello,

    What you could also do is when zeroing at 100 yards while 90degree canted, instead of zeroing it poi=poa you could zero it in a way where the poi is on the sams elevation line as point of aim but horizontally it is to the right the same distance as your scope/bore hight, that way if you scope/bore hight is 6cm your poi should be to the right of crosshairs by 6 cm. What this does is basically makes the line of sight parralel to the bore line and that way you dont have any additional horizontal changes when shooting different distances. Also what i do is create a different rifle profile in my balliatics app with scope hight to zero and juat remember that the poi is 6cm to the right alweys no matter the distance. I shoot mostly using holdovers and this is the way i like to do it and I am not saying this is the proper way it is just simpler for me.
     
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    6.5SH

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    The $12 answer is Strelok Pro. Press and hold the Calculate button, it will change to the HUD screen, scroll to the bottom and can tap left or right 90 degree and learn the offsets the easy way.
     

    Conaso

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