I have some questions about bullet behavoir in flight

[ZeeDoge]

I'm writing down an idea for a game, and need to specify what mechanics will exist and how exactly they will work. One thing I cannot find an answer to is how tumbling works, both in flight and on impact on flesh.

First, let's assume an omniscient shooter that can account for gravity, wind, air resistance, etc firing an average military grade(lets assume not hand loaded) round out of an averagely maintained military firearm. From what I can understand, rounds have some subtle amount of tumbling in flight from imperfections in the round, which can have an effect on flight trajectory, which has an effect on accuracy.

Assuming the above, can the effects of tumbling on flight trajectory be noticed? Is this just the combined effect of other external factors? Can external factors, such as wind or the round slowing down increase the amount of tumbling in flight? If so. how much of tumbling is external factors and how much is it imperfections in the round? And when somebody says that a round is more accurate, do they also refer to the amount of tumbling a round has, or are they just referring to how easy it is to place a shot when faced with external factors? So for example, if you were to program a match round, would you differentiate it from a standard round by it being less affected by wind and air resistance or by decreased tumbling in flight?

Second, what determines what trajectory and direction the round takes when it tumbles inside a fleshy target? Angle of impact? Tumbling in flight? Inconsistence in the fleshy material it is traveling through? Is it something that is better to be just randomized instead of simulated?

 

[Dr.Rob]

They don't tumble end over end in flight unless something is very wrong, cresting a very inaccurate shot. Firing an undersized bullet.. say a 243 in a 308 will get you a 'keyhole' impact, and might also damage the firearm. Hitting a barrier in between the rifle and target, like glass or brush could make a bullet keyhole.

At long range some bullets will experience cavitation. That is the back end of the bullet is spinning in a wider arc than the tip. You can hear such a bullet create a 'bumblebee' sound as it passes by.

Lastly, most fmj military bullets are designed turn 180 degrees in tissue, sometimes breaking at a weak point built in called a 'channelure'. The fired bullet will end up base-forward in a soft target at long rane. At close range it might just poke 2 holes in and out with no yaw at all, or break up. Hitting bone will sometimes make a bullet yaw, fragment or change direction, sometimes not.

There are literally so many factors and chance events possible to create cavitation and yaw that are pretty much beyond the 'average' soldier and average rifle to worry about in training.

 

[45_auto]

I'll try to keep it simple.

To start with, you're confused in your terminology.

"Tumbling" typically refers to a bullet tumbling end-over-end. This is NOT normal during the flight of a bullet. A bullet tumbling during flight is wildly inaccurate, easily a foot or more off target even at 25 yards. And it just gets worse after that!

From what I can understand, rounds have some subtle amount of tumbling in flight from imperfections in the round, which can have an effect on flight trajectory, which has an effect on accuracy.

This subtle "tumbling" you're referring to during flight is more typically called "yaw" or "precession" or "nutation". All three combined are sometimes referred to as "oscillation", and can be typically ignored. These factors are caused by the fact that the center of gravity of the bullet is not perfectly aligned with it's longitudinal spin axis. You can ignore this effect for your purposes. In a typical bullet it will have a minimal effect on accuracy.

You can read more about it here:

http://en.wikipedia.org/wiki/External_ballistics

or in this book, around pages 242 and 243:

http://books.google.com/books?id=Qd...6AEwAQ#v=onepage&q=bullet oscillation&f=false

or here:

http://library.med.utah.edu/WebPath/TUTORIAL/GUNS/GUNBLST.html

Assuming the above, can the effects of tumbling on flight trajectory be noticed?

Assuming you mean "oscillation", it won't be noticed.

Is this just the combined effect of other external factors?

Oscillation is due to the CG not being on the spin axis. The bullet is FORCED to rotate around the spin axis as it moves down the barrel. Once it leaves the barrel it rotates around the CG. In a typical bullet they're so close it is not noticeable.

Is this just the combined effect of other external factors?

No.

Can external factors, such as wind or the round slowing down increase the amount of tumbling (oscillation) in flight?

No.

If so. how much of tumbling (oscillation) is external factors and how much is it imperfections in the round?

No external factors, all imperfections in the round. But it is essentially undetectable in the real world.

And when somebody says that a round is more accurate, do they also refer to the amount of tumbling a round has, or are they just referring to how easy it is to place a shot when faced with external factors?

In a perfect world the "more accurate" round will deviate less from it's intended point of impact. Typically a higher velocity round is easier to place at longer ranges, primarily because it spends less time in flight and the effects of gravity and wind are minimized.

So for example, if you were to program a match round, would you differentiate it from a standard round by it being less affected by wind and air resistance or by decreased tumbling in flight?

At a simple level assuming a linear dispersion and no wind effects, for a typical shooter I would program a standard round to have a random dispersion around the intended point of aim of two or three inches per hundred yards. Assuming a trained sniper and match ammo, I would program a random dispersion of about 1/4 inch around the intended point of aim per hundred yards. You can add random wind effects in addition to those basic numbers. If you want to get more realistic, at longer ranges the dispersion will get exponentially worse.

Second, what determines what trajectory and direction the round takes when it tumbles inside a fleshy target? Angle of impact? Tumbling in flight? Inconsistence in the fleshy material it is traveling through?

All of those and more. A bullet tumbles because it's CP (center of pressure) wants to be behind it's CG (center of gravity). That's why arrows have feathers in the back, and rockets have fins on the back, not in the front. The CP just naturally wants to be behind the CG. A bullet is like an arrow fired feathers first. Given any significant deviation, it will try to spin around so the CP is behind the CG. This is what initiates tumbling.

Is it something that is better to be just randomized instead of simulated?

Professional ballisticians and programmers have been trying to accurately simulate tumbling since computers were invented and haven't been able to do it yet. Literally billions of dollars have been spent on it. If you can simulate it accurately, you'll make Bill Gates look like a beggar.

In other words, for a game, just randomize it.

 

[Al Thompson]

Welcome to THR!

Thought this may be better re-named and moved to reloading.

 

[jmorris]

Reminds me of a time when a friend setup his telescope and we watched bullets on their way to a 300 yd target before. Being directly behind the rifles and the sun at our backs you could see them as they made it to the target, not unlike videos you have likely seen of missles hitting a target.

Some bullets out of one of the rifles you could see "corkscrew" as they traveled. Something that you just can't see under normal situations.