when things go sideways
This post is going to try and explain one of the more difficult combat carbine concepts to comprehend (see what I did there ?). That is the trajectory of a rifle round when firing from the "rollover" or "urban" prone position.
Before I try this, you need to review what the normal trajectory of a fired round is:
While the common phrase used when talking about a bullets flight is that a round "rises" to the line of sight after it exits the muzzle. The fact is that the laws of physics cannot be denied. Like water from a hose, gravity takes hold of a bullet the moment it exits the barrel.
If the bore line and your line of sight were the same, a bullets trajectory would look something like this:
So...the way the sights and the boreline intersect is designed so that the barrel angles upwards in relation to your line of sight.
When the round exits the barrel gravity still takes immediate effect, but instead of starting immediately downward...because of the upward angle of the barrel and the velocity of the bullet fighting the pull, the bullet follows a curved trajectory.
When the rifle is held in this upright position, bullets will impact a target in more or less a vertical string depending on the distance to the target. Wind can impact the strike laterally but that's for another discussion. Within the average engagement envelope of a combat carbine wind is not typically a major concern.
This whole relationship changes when the rifle is canted or held sideways. One of the common positions in modern combat carbine application is the "rollover" or "urban" prone position.
To maximize cover and concealment, it may become necessary to hold your rifle on it's side.
This is when trying to explain things becomes dicey, and if not explained well can leave the student scratching his/her head.
To start with remember these things.
- The relationship of the boreline to the weapons sights remains the same. The barrel still angles "towards the sights".
- The bullets trajectory is no longer an arch. Because the barrel is no longer pointed "UP" to fight gravity, but to the left or right, the bullet exits the muzzle and starts to immediately drop.
- There is no longer an "up and down" stringing of rounds on the target. Bullets will be low and to one side or the other dependent on which side the gun is on.
First. The angular relation between sight line and barrel is the same if it's upright or sideways. What changes is the angular relationship between the barrel and the ground.
Normally the barrel points up and away from the ground:
On it's side the barrel is more or less parallel to the ground. Seen from above, an M4 with it's ejection port up would have a sight/bore/trajectory relationship somewhat like this:
Seen from the side the bullets drop would look more like this:
Not an "arch" but an immediate pull to the ground by gravity.
With the velocities involved this pull isn't extremely "drastic" at shorter ranges (and you are not a surgical sniper). A rifle with a 50/200 zero will be about .5 to .75 inches "low" at 50 yards... 2 to 2.5 inches low at 100. BUT the differences will be drastic the further out you go. 4-5 inches low at 150 yards and at 200 yards you are looking at being 8-9 inches low. Remember, on its side the bullet is constantly being pulled down...no "arch".
Because the barrel is pointed left or right in relation to what side you are laying on, the bullet will continue in that direction till it strikes the ground. A stringing of strikes on target will be similar to this (not mathematically accurate...more as an example):
Remember how at CQB ranges there can be a "sight over bore" issue? A head shot at room distance must be aimed at the hairline to strike in the "sunglasses zone". This is due to the fact that the sights are above the barrel, and the angle of the barrel wont intersect the sight line till it reaches "close zero" which can be 50 yards away. In normal orientation, the rule of thumb is to aim about 2-2.5 inches high for surgical shots from 0-25 yards.
In sideways orientation, the round will NEVER be above your sight line, but left or right of it. The bullet will be striking on the "magazine side" of your weapon at short range by about the same distance as it would have been below your sights in upright orientation.
If you are in rollover prone, ejection port up, and trying to hit a BG between the eyes at 10 yards you will be 2-2.5 inches right (with a 50/200 zero). So at CQB ranges at narrow targets like headshots or knees from under a vehicle it's... "Aim AWAY from the magazine".
For more general combative applications however (read..COM hits under most circumstances), the rule of thumb to remember when shooting rollover is "Aim High and to the Magazine Side".
From 0 to about 75 yards just hold high on the center of the torso.
From 75 to about 150 hold high and to the magazine side around the targets pectoral.
From 150 to 200 hold high and to the magazine side on the targets shoulder
This will get you COM hits only needing to remember three holdovers; 0-75, over 75, and over 150.
There...that's the best I can do. Please let me know if you are confused or if I can clarify anything for you.
I think your first point regarding normal trajectory is one where people get tripped up a lot. They think the bullet is actually rising when it leaves the muzzle, but bullets don't create lift.
There's another interesting factoid that makes perfect sense if you think about it, but that I had never considered until I read it, I believe, in Hatcher's Notebook.
Gravity has the same effect on a bullet's descent regardless of bullet velocity. If you were to fire a round from a rifle over completely flat terrain with the barrel precisely horizontal to the ground and you dropped the same bullet from your hand from the height of the muzzle at the exact moment the round exited the muzzle, the two bullets would hit the ground at the same time. The only variable is how far the bullet will travel while it is falling. I thought that was interesting.
Good point. Yeah I saw something like that too. Physics is physics.
Originally Posted by Devil
I wish I had better animation skills so I could show the difference between the hill-like "arch" of a "normally fired" bullet and the almost "slider pitch" type course a bullet takes when rollover prone....
Originally Posted by tgace
I follow completely but I can see where it would be difficult to explain to someone who hasn't thought it through.
Can you educate us on the urban prone position? Is it for maximizing body armor utility? I still shoot traditional prone, even with pistols. Sideways feels like I'd be limited in mobility quite a bit.
What about this?
Rollover prone/rifle cant is for maximizing cover. If you take cover behind a vehicle you can shoot under it. If you are using a curb for defiled its a technique to use. Even if you are shooting over a object like a car hood it allows you to minimize exposure by staying low....using an RDS for sighting wo having to use cheek weld...
OK, here's a dumb question or two - why don't we see RDS or some other sighting appliance on the sides of rifles? What good are those quad-rails if you're not using them to make you more deadly?
I think you covered this, but what effect does a traditional stock have on "urban prone," position - I mean, it's vertical if you're standing... and if you've made the weapon a "part of your body," even in a sideways position, that orientation must have an effect on bullet travel. See also barrel rise?
I dont quite get ya...
Originally Posted by submessenger
UPRIGHT= Bullet travels UP then DOWN once it leaves the muzzle because the barrel angles up in relation to your line of sight through your sighting system.
SIDEWAYS= Bullet travels out and DOWN...never up, because the barrel is level with the ground and pointing left/right in relation to your line of sight.
And some 3 gunners indeed do put RDS sights on the sides of the AR fore-end, but the ballistics will remain the same in regards to where you will have to aim with that dot. If you are using an optic with mill-dots or a specialized grid-like system you can select specific points to use for aiming.
Last edited by tgace; 1/06/2014 10:25pm at .
I don't know anything about 3-gunners; well, now I know that some use side-mounted sights, thanks for that.
My other question was poorly worded and ignorant. It concerns the physics of how recoil moves the gun, but the recoil shouldn't be significantly affecting the bullet travel, just target (re)acquisition.
True. Recoil only really effects accuracy due to flinching/anticipation before the shot which causes yanking of the trigger. If the trigger is pulled smoothly the bullet will be on its way before recoil could change the impact of the bullet.
Originally Posted by submessenger