[MicroBalrog]

Can anyone tell me where I can find a scheme of the trajectory of a 5.56 round fired from an AR-15/M16A1 rifle?

[The Silver Bullet 1719]

www.ammo-oracle.com

It also has all the other bits and pieces you may want to know.

[MicroBalrog]

http://www.ammo-oracle.com/images/pathshort.jpg


Is that what I think it is? (a not to scale drawing of the trajectory?)

Is there somewhere a more detailed one (with the numbers in metric, if possible?

[Telperion]

If you scroll down about 2/3 of the way (http://www.ammo-oracle.com/#ballmil), there's a table from 0 to 500 yards, at 25 yard increments. Metric ... get a calculator. :)

[MicroBalrog]

So the bullet is climbing out to about 50 meters and then starts dropping?

What makes it climb?

[Telperion]

The barrel is aimed upward in relation to the sight axis.

[MicroBalrog]

The barrel is aimed upward in relation to the sight axis.

So the round itself has no aerodynamic tendency to clim whatsoever?:confused:

[P95Carry]

So the round itself has no aerodynamic tendency to climb whatsoever? Probably minimal and all but immeasurable...... even if bullet is started on its journey with a slight angle of attack .... lift produced will be rapidly negated by the resultant due to downward pull (acceleration) due to gravity.

That diagram shows a trajectory, relative to an imagined sight line ... with 5.56 of course, for 100 and more yards at least, trajectory is very flat ..... so the diag exaggerates a bit..... and seems to be concentrating on a 300 yd shot ...... thus the degree of rise shown from firing.

[ShaiVong]

So the round itself has no aerodynamic tendency to clim whatsoever?

No more than an arrow does :p

[MicroBalrog]

The barrel is aimed upward in relation to the sight axis.

Is that always so, even when shooting at something 25 m away?

In BT, they taugh us to aim slightly below desired point of impact at this distance, why?

[P95Carry]

Is that always so, even when shooting at something 25 m away?

In BT, they taugh us to aim slightly below desired point of impact at this distance, why? With a rifle .. whether scope or iron sights . something that close when sighted on will invariably be below the sight's enforced initial upward elevation of barrel ...... even if that is slight.

It varies from gun to gun.... but few rifles are sighted for this close .... many in fact with iron sights start from around 100 yds so anything closer and bullet is almost certainly being ''launched'' at a slight angle upwards from horizontal.

[MicroBalrog]

It varies from gun to gun.... but few rifles are sighted for this close

But that's how we were supposed to be aiming the rifles BEFORE sighting them! I'm even MORE confused.:confused:



the sight's enforced initial upward elevation of barrel ......

What? I don't get a single word of that.

[Art Eatman]

MB, imagine a barrel whose centerline--axis--is exactly horizontal. Get out your crayola. It helps. Believe me. :)

Upon firing, the bullet instantaneously begins the unavoidable descent of 32.16 ft/sec/sec. One "g", just like any other falling object. (Since the downward velocity begins at zero ft/sec, in one second the bullet will drop 16.08 feet.)

In the case of the bullet, of course, there is a horizontal component to its travel.

Hokay. In order for you to hit a target out at 100 yards, you have to aim the barrel above the target the same amount as the bullet would fall in the time that it takes to travel to the target. If the average velocity to the 100-yard target is 3,000 ft/sec, the bullet will drop 0.1608 feet--or about two inches.

Got that? :)

Next problem: The sights are above the axis of the bore. Now you have a barrel pointed higher than the point of impact, but your line of sight is straight at that point of impact.

Commonly, the centerline of a scope is about two inches above the centerline of the bore of the barrel.

The usual behavior of a .223 (scoped rifle) is that if you're a hair low at 25 yards, you'll be about 1.5 inches high at 100 yards, and right at dead-on at 200. (This ain't perzack, but it'll do.) Maybeso five or six inches low at 300.

So go off and think about all this--with your pictures, and come back with your next question.

:), Art

[MicroBalrog]

OK, I understand even less.

The sight axis slightly above the bore. That would mean that to hit something near me I have to align my sights with a point slightly above the point of impact (thus the bore would be pointed at the POI).

But in fact we where taught to align them with something below the POI at close ranges.

[444]

Where you hold on the target is a function of your zero. The best zero for the M16 series of rifles is the improved battlesight zero as described here: http://groups.msn.com/TheMarylandAR15ShootersSite/zeroingmethods.msnw

You are basically zeroing for 200 yards. The bullet will impact at your point of aim at 50 yards, and 200 yards (approx.).
At any range closer than 50 yards you will need to aim higher than your desired target. To make a head shot just above the eyes, you need to put your sights high on the forehead at 5 or 10 yards.
Think about it like this. If the rifle was perfectly horizontal and boresighted on the target, the sights are going to be above the target simply because the sights are higher than the bore.

Note that we are talking about zeroing a combat rifle. We are not trying to hit pinheads. If you hit someone in the top of the heart or bottom of the heart, the effect is the same. So, we want a zero that makes best use of the trajectory of the cartridge. The IBZ allows you to hold COM from zero out to close to 300 yards. You don't have to estimate range, you hold COM and fire. Head shots obviously require a much more careful shot where you have to allow for the height above bore of the sights and your zero.

[P95Carry]

The sight axis slightly above the bore. That would mean that to hit something near me I have to align my sights with a point slightly above the point of impact (thus the bore would be pointed at the POI). Time to turn in this side of the nation!

Hmmm ..... well with a scope you are right .... you may have to lift muzzle a bit .... this is because of the large difference in axis between scope and bore. Let's tho think iron sights .. for now.

The usual deal with most rifles is that the shortest range the sights will handle is around 100 yds ...... in other words, rear sight is down to it's limit. This might give a suitable POI at 100 but ... at 25 yds, the muzzle is still in effect, pointing up a bit too much, even taking into account the different axes. So ... aim a tad low.

On AR15 of course, even iron sights are set pretty high ...... and so this effect may be lessened...... but still expect the bullet to be launched at a slight upward angle .... to ensure a hit at that 100 yd range.

[MicroBalrog]

At any range closer than 50 yards you will need to aim higher than your desired target. To make a head shot just above the eyes, you need to put your sights high on the forehead.

This is like totally the reverse from what they taught me in the Army!

When zeroing in (25m to actual target) we were told to shoot for the center of the target and to alighn our sight on a point slighty BELOW the desire POI.

Then, after zeroing in for 250m, we were told to hold low on things closer than 50m, dead on for 50m and higher for anything else.:what:

[444]

This is just a different zero. Click on that link I posted. It will describe various zeros used by various branches of the US Military in addition to the IBZ. What you describe I believe is the zero used by the US Army. It has been 20 years since I did it, and I dont remember the details.
The IBZ makes best use of the cartridges' trajectory. But, what really matters is that you know where your rifle hits at the range you are trying to shoot. This is best discovered by you shooting your rifle at various ranges to see for yourself where you need to hold.
All my AR15s including iron sights, Aimpoint, or scope are zeroed for 200 yards and all exhibit a similar trajectory.

[MicroBalrog]

So why do you hold low on things closer than 50m, dead on for 50m and higher for anything else when the rifle is sighted in for 250m?

[gun-fucious]

over time,
gravity is a drag
to compensate
aim higher

[Quartus]

Micro, Art is 100% absolutely correct, but it IS hard to follow without pictures. I'll take a stab at it to try to clear it up. If'n that don't work, I'll cobble together an illustration.


To make it simple, I often find it's best to make sure the basics are covered. So bear with me if I start out with stuff you already understand. (And you never know who might be reading.)


1. Let's take this discussion to Flatland. No curvature of the earth, no atmosphere, no gravity. Just pure trajectory. What will happen? The line of sight is about 2" above the axis of the bore, and parallel. We'll pretend it's 2" exactly. Fire the round (did I mention there's no recoil, either?) and the bullet will fly out in a straight line, and impact the target 2" below the point of aim. Since we tossed out gravity, this will happen regardless of the distance to the target.

2. Let's stay in Flatland, but turn gravity back on. (That's handy, ain't it? ;) ) Don't adjust the sights. Now what happens? Now your bullet impacts much more than 2" below the aiming point. In fact, it doesn't even REACH your 200 meter target. Bummmer!

What do you have to do to hit that sucker? Tilt your barrel up, boy! Tilt it up, I say! Okay, now you are hitting your target. But what path is the bullet following?

Well, remember that it starts out 2" blow the line of sight (LOS). But the barrel is tilted upwards, so it follows an arc that takes it ABOVE the line of sight, then drops back down and (we hope) intersects the LOS right at the target. So if it started out BELOW the LOS, and then goes ABOVE the LOS, and then comes back down to MEET the LOS, we can see that at some point close to the muzzle it must have CROSSED the LOS.

It happens that for the M-16, that first crossing of the LOS happens at 25 meters. Now, stick a target at that distance and what hae you got? A 25 meter zero point that puts your round on target at 200 meters, too. (Er wuz it 250?)

So there it is. Your bullet starts out BELOW the LOS, rises ABOVE it (crossing it on the upward part of the trajectory at 25 meters) and then falls back down to the LOS at the 200 meter target. You zero for 25 meters to take advantage of the trajectory.

With a rifle that has iron sights right on the barrel, this phenomenon isn't seen because the first crossing is right there in front of the muzzle. We see it with the M-16 because the sights are so high above the bore.


Clear as mud? :D


Now, I've been a long time typing this, in between phone calls, so I hope nobody has gotten on this thread and posted that the 5.56 "flies" like a plane or rocket because it travels so fast. :rolleyes:

IF any one has, please go back and start over in 5th grade science, and come back and join the discussion after you complete some high school physics. You are just flat WRONG.

[444]

That graph you posted, makes all this pretty clear (to me). By the way, it is in metric.
If you notice,when using M193 (55 grain bullet) the center of the left side is 0.0 meters. This is your line of sight. Note that the line crosses 0.0 at a point just shy of 50 meters (which would be very close to 50 yards). Before that point, the bullet (the line) is below 0.0 (your line of sight). Between 50 meters and 240 meters (approx.) the bullet will be higher than your line of sight. Beyond 240 meters (approx.) the bullet will be low.

So, if you zero your weapon to hit point of aim at approx. 40 meters, the bullet will stay within a 4" zone from zero to about 275 meters (approx.). So, you hold COM from zero to 275 meters you hit within 4" of your point of aim. Even at 300 meters it is only about 5" low. Also note that at a range of approx. 5 meters, the bullet will strike approx. 2 inches low. At approx. 25 meters, the bullet will strike approx. 1" low.

[ShaiVong]

Hey! Think of the bullet as a flying fish Micro! Your aiming at the same level as the top of the water, but the fish starts right below! It goes up up up out of the water (crossing your sight line!) then glides a bit before coming back down!

Thus, your point of impact is exactly where your sight line is twice! It starts impacting low, then impacts right where you want it to, then high for a bit, then zero again, and finally low again!

FISH! :D



I say, I say, don't they teach you physics down there in the desert boy?

[MicroBalrog]

At approx. 25 meters, the bullet will strike approx. 1" low.


So why do I have to HOLD low when aiming at 25m?:what: Wouldn't that make it strike even lower?

[Al Thompson]

Your instructors are mis-informed. Happens here too. :)

[MicroBalrog]

Your instructors are mis-informed. Happens here too.

But their instructions work. How?:confused:

[Art Eatman]

MB asked, "So why do you hold low on things closer than 50m, dead on for 50m and higher for anything else when the rifle is sighted in for 250m?"

The only thing I can think of is that the bullet is crossing the line of sight at somewhere around 20 or so yards.

If the rifle is zeroed for 250, you're gonna have to hold some two or three inches LOW to have a target-center POI at 100 yards, since the bullet is above the line of sight at that distance...

Again: The barrel is pointed slightly upward, and the trajectory curves to the desired 250-yard zero. The line of sight is direct. The bullet will be above the line of sight from around 20 to 25 yards until 250.

When the bullet is above the line of sight, you MUST hold low in order to hit a specific point. Ya gotta "bring the bullet down" from above the desired hit-point.

Art

[P95Carry]

MB ...... most has been covered by now and pretty well by many ..... but I remember on another board when someone had a prob getting the picture of it all ..... I drew a diagram .... just to clarify what one has in mind when discussing this. It exaggerates a lot .... but just to make a point.

Take a look and see if it helps a bit more ... it may not solve your dilemma re the 25 yd situation but this is a subject where perhaps any and all bits of info help toward the goal of a solution.

http://www.thehighroad.org/attachment.php?s=&postid=523299

[MicroBalrog]

I got it!

On the Israeli standard zeroing target, there is a mark for and ideal M-16A1 POI.

According to the U.S. M16A1 manual, on an ideally zeroed rifle, the POI is 2.4 cm below the Point of Aim. The ideal IDF POI was about 5 cm below target center. We were told to aim 2 to 2.5 cm below target center. So they were right, but their ration was wrong.