Gun Fans - M21 - Aiming BELOW target?

[Guest]

Why is that? If a target it very far, you can still hit it by aiming below the target. Just curious why this is - is this accurate or just a game thing?

[Mister Frag 0]

Did you read page 13 of the OFP manual on how to use the scope? It works just fine...

[Guest]

Nope, can't say I did. Gotta look that up...

p.s. There's a MANUAL??? Just Kidding...

[Mister Frag 0]

The thing to keep in mind is that almost all firearms use a setup where the bore axis and sight axis intersect at some distance.

With the sight axis level, the bore axis will actually be slightly elevated, and the bullet will travel upwards for some distance after leaving the barrel before it starts descending due to gravity (the slight upwards angle also imparts some lift on the bullet, but is is negligible).

Anyway, at some point during climbing stage of the trajectory, the bullet passes the the sight axis, and then again during the descending stage. The second intersection of the bullet trajectory with the sight axis is what is commonly known as the 'zero' of the sight or scope -- this is the distance at which the bullet hits the intended point of impact.

With a non-adjustable sight or scope, there is only one zero, but there are scopes that have a Bullet Drop Compensator, or BDC. The BDC is matched to the bullet and its external ballistics (the stage between what happens inside the barrel and inside the target), and can compensate for the distance to the target, allowing the point of aim to be the same as the point of impact over a certain range.

In the case of the M21, the scope must be calibrated for the distance to the target as described in the manual. If that is done correctly, the point of impact will be in the center of the crosshair reticle regardless of the distance to the target.

(Mister Frag gets off his soap box)

[DarthBeavis 0]

Mister Frag that is bullsh*t and you know it. Just admit the truth. It is all magic and pixie dust

[Mister Frag 0]

d*amn, I hate it when someone calls my bluff!

[Guest]

Mr. Frag - you are da man. All this time and I thought the +/- keys were just for zooming. Turns out those little notches do have some meaning...show you how much I know about guns. Oh well. Looks like more manual reading for me...

Thanks again.

[RedRogue 0]

Also the type of sighting system for the scope on the M21 and the SVD is zeroed in for a firing plane perpindicular "level" to gravity. If you shoot up a hill your bullet will undershoot your target when zeroed in for a normally level plane. And shooting downhill will of course over shoot your target when zeroed in for what would normally be a level firing plane. If you wish me to go into more detail with this I can, but its very late right now....zZzZzZzZzZzZzZ.

[Mister Frag 0]

</span><table border="0" align="center" width="95%" cellpadding="3" cellspacing="1"><tr><td>Quote </td></tr><tr><td id="QUOTE">Quote: from RedRogue on 5:47 pm on Dec. 14, 2001

Also the type of sighting system for the scope on the M21 and the SVD is zeroed in for a firing plane perpindicular "level" to gravity.  If you shoot up a hill your bullet will undershoot your target when zeroed in for a normally level plane. And shooting downhill will of course over shoot your target when zeroed in for what would normally be a level firing plane. If you wish me to go into more detail with this I can, but its very late right now....zZzZzZzZzZzZzZ.<span id='postcolor'>

Actually, that's not correct, in both cases you will have to adjust the point of aim down. The reason is kind of hard to grasp, but I'll try to explain it.

Say you have target that is 300 meters away, and at 0 degrees elevation. When you fire, the bullet will drop over the distance of the target, which is 300 meters. This much everyone agrees on.

Now lets say that the target is still 300 meters away, but at an elevation of 45 degrees. The horizontal distance to the target is now only cos(45) * 300 = 212 meters. Why is this important? Because the effect of gravity is only relevant on the horizontal vector. Since the horizontal vector is now shorter than it was in the first case, you will have to aim lower. The same will still hold true even if you aim down at 45 degrees, because the horizontal distance is still the same.

I know this doesn't seem to make sense, so if you have questions, ask. I tried to be as clear as possible at 2:01AM, but I'll probably be horrified when I read this later.

[Guest]

That does make sense. Which is why my original question was, why do I need to aim below the target. What I didn't realize earlier was that my sniping position was almost always at a higher elevation than the target. So, even if I am calibrated - is that the word? - I would still overshoot, which is why I needed to aim below the target.

[Mister Frag 0]

OFP is supposed to have a realistic ballistics model, so this could explain it. The 45 degree example is a bit exaggerated, but it helps to explain what happens when you engage a target that is not at the same elevation.

Obviously, with lesser degrees of elevation, the difference can be negligible -- at 10 degrees the distance is cos(10) * 300 = 295 meters, which won't really affect the point of aim.

[Guest]

<SARCARM>

Dammit - it's just not realistic enough. It doesn't account for bullet fragmentation after hitting the target. BIS is crap!

</SARCARM>

<SERIOUS>

This is a d*amn good game

</SERIOUS>

[mads bahrt 0]

There could be another reason for aiming low being so effective: When i was in the amry we were told to aim low when we were in doubt on the distance. There is 2 benefits from that: You can see were the bullet strikes from the small dustcloud the bullet makes when it hits the ground in fron of the target. Second there is quite a big chance that the bullet jumps up and hits the target after hitting the ground. I don't know if BIS implemented this, but to me it seems so.

[Mister Frag 0]

And subsequent shots might hit the target if you are shooting a rifle in burst mode or full-auto due to muzzle climb resulting from the recoil.

[RedRogue 0]

Gravity is only relevant to the vertical vector, not the horizontal vector.

In a ideal condition with wind resistance removed from consideration where you fired a rifle perfectly parrallel along the horizontal plane which is perpindicular to the field of gravity. The velocity (say 1000m/s muzzle velocity) would be completely in the horizontal vector, this velocity would remain unchanged until the bullet was stopped by impact with the ground. The vertical vector is influenced by the acceleration due to gravity of 9.8 m/s^2.

So lets say you fire this rifle parrallel to the horizontal plane 1m off the ground. Since y=y{initial}+v{initial}t+(1/2)at62 time would equal the square root of 2y/a. now y being the distance from the ground, a being the acceleration due to gravity and t being time. The time it would take the bullet to hit the ground is .45 seconds. Since the bullet has continued to travel with a horizontal velocity of 1000m/s during this time it would have traveled 452m from the rifle until impacting the ground.

Now say you want this same rifle to hit a target that is 1m off the ground 452m away being fired from 1m off the ground. Naturally the rifle with have to be fired at a angle from the horizontal plane now to have the bullet rise then drop do to gravity to hit its target. This angle of the barrel due to range is what is already taken into account in the optics of the scope when fired at a target that is at the same elevation as you.

Now here is where it gets tricky, I did leave the math out of the paragraph before but this here applies to it as much as it does here. But the scope is already calculated out for a target at the same elevation as you.

Mr. Frag you are right that when firing at a target with an elevation angle greater or less than 0, the horizontal vector distance is less than the distance of a 0 degree elevation. But your horizontal vector velocity is also lowered.

So firing at a target 300m away at an elevation of 45 your horizontal distance is only 212m away which you are completely correct on. But your horizontal velocity cos(45)*1000m/s= 707.106m/s and you now have a intial vertical vector velocity of sin(45)*1000m/s= 707.106m/s. Now this would be all fine if there where no such thing as gravity, but due to gravity you would have to further incline the barrel to negate the negative acceleration of gravity on the vertical velocity so that the bullet would hit its target and not "hit the hill" under it due to the acceleration by gravity. This would further increase your vertical vector velocity and decrease your horizontal vector velocity.

Now I know this is hard to follow thru mear text, and I might not be explaining it all to well. But this is why on a positive elevation angle you must shoot above your target and on a negative angle shoot below your target given the nature of the scope in game. The scope given in the game is calculated for a 0 angle of elevation shot in which the bullet rises a bit then falls to hit its target, any further positive angle the bullet follows an arc, first rising then falling to its target. On a negative angle shot the bullet only falls to its target following a sloped path. Now in real life most good scopes as is the real M21 scope are made to where you have elevation "ticks" where you set the scope to be "recalculated" for a elevation angle greater or less than 0.

[Mister Frag 0]

The force of gravity is a constant over time, and since the distance to the target doesn't change, the effect that gravity has on the trajectory doesn't change.

Rather than trying to explain this in my own words, I will use illustrations from the excellent Long Range Shooting Tutorial by Karin Christensen, available at

  <a href="http://www.shooterready.com

The" target="_blank">http://www.shooterready.com

The</a> images are some of the ones that cover external ballistics, which is only a small part of what the tutorial covers.

(Edited by Mister Frag at 12:27 pm on Dec. 15, 2001)

[Charlie_McSheenie 1]

gravity is a b*tch

i've always wanted to say that

(does some quick crossing out on his life goals list)

[HuFlungDung 0]

RedRogue, I think where your theory slipped up, is in this bit of logic:

Quote:

The scope given in the game is calculated for a 0 angle of elevation shot in which the bullet rises a bit then falls to hit its target, any further positive angle the bullet follows an arc, first rising then falling to its target. On a negative angle shot the bullet only falls to its target following a sloped path

End Quote.

Although what you say is true, the line of sight through your scope has not changed relative to the bullet arc, and predictable aiming makes everything relative to that line of sight, and not to the earth's plane of reference.

The bullet drop is consistent for a given horizontal vector. Because shooting out of level always decreases the horizontal vector, the time that acceleration due to gravity has to act, is shorter. This means that the bullet will seem to be higher than normal,  because it is "earlier"(so far as accel due to gravity is concerned) in its trajectory.

(Edited by HuFlungDung at 2:08 pm on Dec. 19, 2001)

[RedRogue 0]

I had already made a post saying this, but I guess the forum decided not to keep it.

Mr. Frag is right, it will aim high either up or down. I neglected to calculate the time from the resultant vector. While the horizontal vector of velocity does decrease, it does at a slower rate than the horizontal vector of distance. So yes an elevation angle will result in less time for gravity to act on the projectile, causing the weapon to hit high.