Trajectory of Bullets
August 24, 2007 9:09 PM   Subscribe

would the trajecetory (twist) of a bullet shot from a handgun (Luger) change if the gun was shot in the right hand of the shooter versus the left hand of the shooter assuming everything else remained exactly the same?

Disclaimer: I am not planning to shoot anyone!!! My husband is the defense lawyer in a murder case and we were discussing this issue. Thank you in advance for your input.
posted by Lylo to Science & Nature (22 answers total)
posted by lastobelus at 9:13 PM on August 24, 2007

Not even a little bit.
posted by chimaera at 9:14 PM on August 24, 2007

You are conflating two different things, trajectory and rotation. A bullet's rotation is acquired from the rifling in the barrel of the gun, which would be the same regardless of the hand in which the gun was held. The trajectory of a bullet would be determined by the type of gun, the type of ammo, and all sorts of other factors, handedness not being one of them.

On preview...uh, no.

posted by BitterOldPunk at 9:22 PM on August 24, 2007

The rifling inside the barrel remains the same no matter what hand you shoot with. It's sort of like asking if the toilet would go counter-clockwise if you flush with the left hand.
posted by Addlepated at 9:32 PM on August 24, 2007 [1 favorite]

Thank you BitterOld Punk, you are right, it is a rotation issue, I threw in the word trajecory myself and didn't know what I was talking about. Expert witness is saying that because retrieved cartridge had a left twist this indicates shooter was using his left hand. Do you see any possible basis for this opinion? Thanks so much for your input.
posted by Lylo at 9:33 PM on August 24, 2007

GREAT analogy, Addlepated. Thanks!
posted by Lylo at 9:37 PM on August 24, 2007

An expert witness actually said that? Wow. That's so ridiculous. What was he/she an expert in--underwater basket weaving?

If that were true, I wonder what sort of twist the cartridge would have if the shooter had held the gun in both hands. Or laid it on the ground and fired it with his foot. Or attached a string to the trigger and fired it remotely.
posted by cerebus19 at 9:43 PM on August 24, 2007

I hope the defense can afford another expert, because you're going to need one to respond to (or even understand) what this one is saying. IANABallistics Expert, but is the expert saying the cartridge (i.e. the brass part of the shell that gets ejected from the breech of the gun) had some sort of "left twist," or that the bullet had? I can imagine that the brass might have some sort of tell-tale scratching that was different between a left- and right-handed shooter, but I would be surprised if the bullet was distinguishable.
posted by spacewrench at 9:48 PM on August 24, 2007

no, why would it? the gun doesn't change, only its location does. you might get different trajectories, but not rotation.
posted by thinkingwoman at 9:48 PM on August 24, 2007

If it is indeed an expert witness, and he/she is talking about the cartridge and not the bullet, then it likely has something to do with the manner in which the shells were loaded, which may produce microscopic scratches that vary with handedness.
posted by weapons-grade pandemonium at 9:59 PM on August 24, 2007

Expert witness is saying that because retrieved cartridge had a left twist this indicates shooter was using his left hand.

Wait... he claims cartridges acquire a "twist" which is determined by which hand the shooter uses? The cartridge doesn't even pass through the barrel of the gun! It can't be affected by barrel's rifling.

On the other hand, guns often have features that might result in left-handers tending to hold them in a different grip than those preferred by right-handers, and I suppose that this could affect the path of an ejected cartridge. Perhaps he was speculating that the position at which the spent cartridge was found relative to the location at which the gun was fired indicated a likelihood that the gun was held in a manner more comfortable for a left-hander. Seem a stretch to me, but I'm no gun expert.
posted by RichardP at 10:07 PM on August 24, 2007

Bullet and cartridge are two different things! The trajectory of the bullet would, as we've established, not change.

Markings on the cartridge could be different if the gun was designed for left-handed use, however.

After firing, when the action cycles open, the spent cartridge is ejected and usually (in a semiautomatic) the next round feeds into the chamber, and the action cycles closed. In most guns, the ejection happens to the right, which is fine if you're right-handed but for southpaws results in a face full of brass.

A left-handed gun, then, ejects the cartridge to the left. Except, how would you tell by looking at the cartridge?

I'm not a gun expert but I do know that when you squeeze the trigger, a series of levers moves a hammer back, which a spring then throws forward against the primer, which ignites the powder and fires the round. The hammer dents the primer, and presumably the dent would tell you which way the cartridge was oriented when it was fired.

By examining scrapes along the cartridge's body, and comparing to the primer dent, you might be able to tell whether the ejection port was on the left or right of the body. That would tell you whether the gun was made for a right or left-handed shooter, but not how it was actually held when the trigger was pulled. Lots of lefties learn to shoot right-handed because specially-built guns are more expensive.
posted by Myself at 10:35 PM on August 24, 2007

General rifling characteristics found on a fired bullet (specifically - direction of twist and number of land and groove impressions) are class characteristics imparted by the firearm from which it was fired. It has absolutely nothing to do with the handedness of the person who fired it. Likewise, as previously mentioned, direction of twist would be found on the fired bullet and not a fired cartridge case.

If this a government witness working in an ASCLD/LAB accredited laboratory, my guess is that this is just a misunderstanding. God, I hope so.

There may, however, be other things that might lead one to believe a shooter fired with one hand over the other. Gunshot residue, chemical testing for trace metals (Ferrotrace), or an injury on the hand all might support one theory over another. Still, none would necessarily be conclusive of the shot in question being fired by one hand over the other.

If it's a real issue in the case I'd recommend getting your own experienced defense witness.
posted by whatisish at 11:03 PM on August 24, 2007

A left-handed gun, then, ejects the cartridge to the left. Except, how would you tell by looking at the cartridge?

Recoil operated pistols sometimes leave a "drag mark" that extends from the firing pin impression. It occurs as the barrel cams down from the slide during the rearward travel of the slide. This drag mark normally occurs at the twelve o'clock position. If the extractor mark is at approximately 9 o'clock and the ejector mark is at approximately 3 o'clock, then that would be consistent with a left handed ejection port. Still, as you mention, it really has little relevance since the shooter could still have fired it with either hand.
posted by whatisish at 11:13 PM on August 24, 2007

Then again, I can't think of a left handed pistol off hand. There may be some - certainly not very common. Left handed long guns aren't so rare... and Charter Arms produces a left handed revolver - but that wouldn't eject the fired cartridge case.

Ok, now I'm done.
posted by whatisish at 11:31 PM on August 24, 2007

Thank you BitterOld Punk, you are right, it is a rotation issue, I threw in the word trajecory myself and didn't know what I was talking about. Expert witness is saying that because retrieved cartridge had a left twist this indicates shooter was using his left hand.

As far as I know, cartridges don't get a 'twist' at all, just bullets. And if he's talking about the bullet... holy CRAP this isn't true. Bullets twist because of the rotation of the barrel. The way to cross examine: show that the gun barrel has a left twist. Ask the "expert" if the bullet would have a left twist if held in the right hand. Ask the "expert" if the bullet would have a right twist if the barrel had a right twist. If he's talking about the bullet, he has NO FUCKING CLUE, and it should be easy to dismantle and discredit him.

Markings on the bullet and case tell you about the GUN that fired it, but nothing directly about the finger on the trigger.

Conducting your own firing tests with a similar weapon would probably also be a good idea, although I don't know if that would be allowed.
posted by Malor at 2:21 AM on August 25, 2007

Is it possible that he is saying something about the trajectory of the ejected cartridge? I am under the impression that a handgun held in the right hand will recoil (slightly) to the left, and I assume the reverse is true for a handgun held in the left hand. (I've only fired a handgun one-handed once, we were told to fire both-handed, so ...) Since the cartridge is being ejected from (now moving) handgun, it could theoretically have a different trajectory.

Of course, I have no idea how you could tell after the fact that this was the case.
posted by Comrade_robot at 3:53 AM on August 25, 2007

bullet + casing = cartridge.
posted by Brian B. at 9:32 AM on August 25, 2007

I think the expert must be talking about the trajectory of the casing.

Because the riflings in the barrel give a spin to the bullet, an opposite spin, roughly speaking, is imparted to the gun. If the gun gives a clockwise spin to the bullet (from the point of view of the shooter) the gun will twist counter-clockwise in the hand of the shooter.

For a right-handed person, this means the gun will twist into the palm, for a left-hander, the gun will twist against the fingers. The right handed person would seem to have an intrinsic advantage in keeping the gun from rotating over the left-hander.

So when the casing is ejected, the gun would tend to be more over on it's side when fired by a left-hander, and that might affect the trajectory of the casing.

This effect appears to me to be rather weak and likely to be overwhelmed by many other factors in any given real situation, however.
posted by jamjam at 10:15 AM on August 25, 2007

Comrade_robot; this is something about physics that may not be initially obvious, but when an object leaves a moving path, no matter what direction that path was going in, the object proceeds in a straight line.

So, if you were swinging the gun around, the bullet would still leave the gun on a straight path from wherever the gun was when you fired it. It wouldn't "curve."
posted by odinsdream at 11:20 AM on August 25, 2007

The simple fact that a gun is fired by one hand or the other does not directly affect the trajectory of the bullet fired from the gun. As someone else pointed out, it clearly cannot affect the rotation direction of the bullet or the marks left on it, as those are imparted by the rifling within the barrel.

However, if everything is exactly the same, including the handed-ness and eye dominance of the shooter, then those factors may affect the shooter's firing of the gun, and thus the point of impact.

For example, the fine motor skills of a person's off hand (the left hand for a naturally right handed person) will tend to be less smooth unless the person deliberately trains the off hand. For shooting, this results in a tendency to "jerk" the trigger (pull the trigger roughly, typically resulting in the point of impact moving downwards on the target as the "fist clench" causes the muzzle to depress towards the ground). For an easy example, try brushing your teeth with the hand other than the one you usually use. Odds are the brush strokes will be jerkier and the brush will feel hard to "aim" correctly.

A poor or unfamiliar grip on a handgun can also contribute to "limp wristing," in which the gun is not held tightly or the wrist is loose. This typically contributes to malfunctions of semi-automatic handguns rather than affecting accuracy of the fired bullet. The stiffness of the hold of the firearm can, however, affect the trajectory of the ejected empty shell casing. A weaker wrist will absorb some of the energy used to cycle the action of the gun, reducing the velocity of the slide (the big piece on top that reciprocates back and forth), and thus altering the path the ejected case takes. E.g., a weaker hold may cause the case to "dribble" out and fly just a few feet versus flying 10 feet or more from a firm, solid hold (where "hold" here refers largely to how well locked the wrist is to resist the back and up rotation that the firing of the gun imparts). I'm not familiar with the forensics of this situation, so I couldn't say how the difference would affect the marks on the ejected case.

Eye dominance can affect the point of impact of the bullet for cases of cross eye dominance, i.e., looking through the sights with the non-dominant eye. This can result in severe misalignment of the sights causing shots to fall far to the left or right of the intended point of impact. To illustrate, hold a finger up at arms length, with some object (the "target") behind, in the distance. Focus on the fingertip. Without changing focus, notice that there are two "ghost" images of the target object in the distance. Normally, the dominant eye naturally lines up behind the "correct" image of the target. In cross eye dominant cases, e.g., a right handed shooter looking through the sights primarily with the left eye, the shooter may inadvertently line up behind the "wrong" image of the target. This type of problem is most easily resolved by closing one eye (typically the eye opposite the side of the hand holding the gun, so a right handed shooter closes the left eye).

[disclosure: I am a firearms instructor]
posted by doorsnake at 1:44 PM on August 25, 2007

thanks to everyone who answered. All of the technical aspects really helped, Thank you, the defense is now much better armed (so to speak). However, in terms of talking to a jury in terms they will understand, Addelpated's analogy about flushing a toilet is PERFECT and we are expecting to hear a lot of toilet flushing come out of the jury room bathroom during deliberation. I love metafilter. Thank you again.
posted by Lylo at 10:41 AM on August 26, 2007

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