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Zeroing your rifle? Ballistics question

Discussion in 'General Infiltration Discussion' started by Keganator, Nov 6, 2001.

  1. Keganator

    Keganator White as Snow Moderator

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    whups...lemme look some more...
     
  2. JamesT

    JamesT sniper apprentice

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    As for Magnus lift, it has little effect on bullets when there is no strong side wind.

    The most effective factor which causes the bullet to fly upwards is the exit angle. If the exit angle is a true zero degree, perpendicular to the gravitational field, the lift can be gained by aero dynamics, if any, is almost none.
     
  3. Keganator

    Keganator White as Snow Moderator

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    Okay then. So, assuming no side wind, there's no magnus effect. That makes it much simpler.
     
  4. })FA|Snake

    })FA|Snake New Member

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    um... wouldn't you zero it for the range you intend to be shooting it at?
     
  5. yurch

    yurch Swinging the clue-by-four

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    yes, but that would require extreme accuracy in setting the proper angles. Its not just some magic dial you turn, a gun has to be carefully sighted in in the first place.
     
  6. Tiffy

    Tiffy Back to champion the L85

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    When your in combat you Zero your weapon using whatever is available. In the British Army you'd zero your weapon at 100m and then check fire it at 300m (thats for a battle/assult rifle, sniper rifles might be zeroed at a longer range, but not over about 300m). In the Falkland we zero'd weapons every day if we could, always every other day, at ranges from a little as 50m.

    When you zero a rifle you aim at the centre of the target and depending on range the group should be centred a set height above or below the centre. If I remember correctly (its to late to try and find the referance to check this at present) an L1A1 SLR 7.62 Rifle when zero'd at 100m should produce a group 4" above the target centre when using the 300m range setting on the sight.

    With the L85 and is SUSAT you can Zero the weapon with out firing a shot using a collimator. You need to have zero'd the weapon in the usual way to get the collimator setting but after that you can reset the sights back to this setting without firing a shot.
     
  7. yurch

    yurch Swinging the clue-by-four

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    Any info like that on the other calibers?
    And how fast (per second) will a bullet in-flight slow down?
     
  8. Tiffy

    Tiffy Back to champion the L85

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    You need to know the drag coefficient( Cd) for the round in question to know how fast its going to slow down, as well as if its supersonic or not. Bullets that are supersonic slow at a higher rate due to having a higher Cd.

    A simple formula for calculating Cd is :-


    Cd =
    (8 * Drag Factor) /
    (Fluid Density * Velecoity sqd * Frontal are of the round)

    The Drag Factor is got from experimental results.

    Ballistics is really a quite complex subject and if its going to be simulated in INF will need simplifing somewhat.

    As a guide the Cd of a 7.62mm NATO Rd is approx 0.12 upto Supersonic Speeds.

    At Mach 1 it increases to about 0.45 as the Drag Factor is very large at this speed.

    It then starts to fall and at M2 is about 0.3.

    I think we'll just have to settle for an approximate solution for INF cause thats a lot of calculations for each bullet otherwise.

    Think I'll stop here as I'm starting to prattle and I think I've probably confused most of you by now :p
     
  9. FiXiT

    FiXiT New Member

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    Ok just my 2 cents, in case anyone is interested:

    I don't know about all the equations and all but a lot of bullet drop ballistics can be found by contacting the ammunition manufacturers (Remington, Federal, etc.) They're also often printed on the sides of ammunition boxes and available through other printed formats. These BD matrixes often specify BDs for a number of different zeros. Let me know if anyone really wants them (I should have BDs for every major caliber and weight).

    Now about zeroing. I've heard about marine snipers zeroing somewhere up to 600yards out but beyond that I'd think it impractical and sometimes physically impossible. Most scopes have internal mechanisms that allow you to move its reticle relative to the line of the barrel & scope but there are limits. Actually using cants or wedges so that the scope itself is mounted at an extreme angle may allow you to zero at 1000yards but then, at 100 yards, you might have to place your scope recticle so low that the target is actually out of sight. Furthermore its almost impossible to check your zero at 1000 yards.

    Practically, as a hunter, I'd zero at about 100yards. This process involves mounting the scope and boresighting it at 25yards (looking through the barrel and looking through the scope and ensuring they point to the same spot). Then you shoot a couple of rounds to confirm. Then using a bullet drop chart you adjust your sight to zero at 100yards or 300yards. You do this by referring to the chart - if it says that with a 25yard zero the bullet will fall 5 inches below the point of aim at 200 yards then you crank your sight up about say 9 clicks (2.5MOA) maybe. Then you shoot again to confirm that you indeed are zeroed at 200 yards and adjust accordingly. This last check makes it impractical to zero at 1000 yards because you can't see your bullet holes at 1000 yards.

    In fact most practical rifle scopes use about 10 -12 power maybe. Any higher and you may get mirages. Even with a 12 power scope its impossible to see bullet holes at 300yards. Usually when I do zero out that far I have to use a metal swinging target and listen for the ping. Sometimes it can be frustrating when you don't hit it at all and theres no way to tell if you're shooting high low etc.

    Btw just fyi, even if you've perfectly zeroed at 1000 yards, if you're aiming a whole degree (60 minutes) off target (like if your hand shakes) then your bullet is going to hit some 50 feet off target at 1000 yards.

    Last thing, nice illustration Yurch, sometimes I have a hard time explaining to some people that the bullet doesn't really gain lift in the first half of its flight (but you said it hits high right?), its simply that the zero is at a further range and you're aiming the barrel up as a result. Ha ha.
     
    Last edited: Nov 11, 2001
  10. yurch

    yurch Swinging the clue-by-four

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    The reason why we ask this, is that we are trying to get a basic ballistic model with the gun zeroes modeled in. I got the bullets to fly in a basic arc the other day... don't have time to work on it now however. :p
     
  11. JamesT

    JamesT sniper apprentice

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    For bigger caliburs' ( 5.56 aka .223 Remington, and above ) ballistic data, you can find them here.
     
  12. yurch

    yurch Swinging the clue-by-four

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    yes!
    Thank you.
    Do you know what we could use now? A map with an agreed-on scale for testing... Anyone know of one, or can make a quick one?
     
  13. JamesT

    JamesT sniper apprentice

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    As for range determination, I really have no idea.

    I think Beppo is the only and the ultimate answer.
     
  14. JamesT

    JamesT sniper apprentice

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    This site provides a very complete online look up system for bullet data, including almost all the trajectory data you need ( in fact more than enough ). Only that you have to spend some time to dig those data out via that system.
     
  15. Tiffy

    Tiffy Back to champion the L85

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    Beppo's code is commented with these measurements.

    16 units = 1 foot although he is using 48 units = 1m.

    Hope this helps.
     
  16. Keganator

    Keganator White as Snow Moderator

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    Yurch: just make one! It's not that hard; a subtracted square, some light, and measure out 16 UU = 1 foot, and it'll all be good.
     
  17. yurch

    yurch Swinging the clue-by-four

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    Umm. I have never used the map editor. Ever.
     

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