Briman

People in gunshops are human too. If they don't know any better they will not only subscribe to but will also spread such myths as brush busting bullets, rising bullets, and bullets/calibers that have so much energy that they will litteraly knock 200 lb animals, 2 legged or 4 legged, over after the animal is completely flung in the air.

Joe D.

I was with Barnes yesterday when we went to the gun shop. I had Barnes post this because he is a re-loader and he knows a little bit more about ballistics than I do.

I wanted to get a scope mounted with lower rings because the old set-up had see-through mounts and when sighting it in at 50 and 100 yards, the difference of point of impact was pretty different. We posted the problem on the board and we got a lot of explanations on why this was happening. Yesterday when I explained to him the problem with the mounts, he said that it shouldn’t be an issue (that’s another can of worms). We got into a discussion about sighting in scopes and he made a comment that in warmer weather, the heat rising from the ground will cause the bullet to rise (maybe I should of asked him if a bullet sinks faster because of the cold air, just to get a laugh). I don’t know much about ballistics, but I told him that a bullet only rises due to the barrel being pointed up and that it never breaks the plane of the barrel, he disagreed and gave me this air pressure theory and something to due about a vacuum the bullet causes when being fired from the gun. Almost appeared that the bullets he fires have wings and a flight control stick.

Sad thing is, this guy mounted my scope, however, I think he knew what he was doing with the mounting part. He’s one of those guys that work at a mechanics shop and only changes and balances tires; occasionally he may change the oil.

pyral

With idiots like this that have no clue on ballistics, I can almost see how other idiots came to believe "guns kill people"-- Air-Bullet-Control simply had the wrong coordinates and misdirected the flight paths!!! []

Nomercy

This is really technical, and pretty picky, and I really don't like applying my job to my pleasure, but I've got to comment.

My granddad used to have a saying "don't speak on what you don't know"....in this case, you are both guilty.

The guy is wrong because of his reasoning...it isn't exactly air currents that raise a bullet. That is a myth, very similar to the "bullet shedding myth", supposedly bullets sand themselves on the air as they fly because they are so fast, which is bogus. But I digress.

You are wrong because this actually CAN happen. It's called the magnus effect. Although I've never studied projectiles like bullets for this effect, I have studied the analogous scenario of a spinning rod (perpendicular to fluid flow) in a pipe of flowing fluids. If the pipe where stationary, two basically symmetrical "boundary layers" of static fluid would form on either side of the rod, in a fairly rapid flow, these layers detach in what is known as vortex shedding, which caused the failure of the Tacoma Narrows bridge in case you're curious....It's also why a flag whips back and forth in the wind.

However, if you start spinning the rod, these boundary layers become uneven, producing a velocity difference between the two "sides" of the rod, which creates a pressure difference similar to that of an airplane wing. So, this is VERY minimal in air because it is a low viscosity fluid (yes, it's a fluid, even if it isn't a liquid).

For example, if you're looking at the tail of your bullet in flight, and you have a strong wind left to right, and your bullet is rotating counter-clockwise as viewed from behind, this will induce a more rapid drop in your bullet...the same bullet in a right to left wind would raise.

It's a real thing given the right conditions. It wouldn't happen in still air, and it likely gets attributed to the wind any way if it were to be noticed on a windy day...the bullet drift horizontally with the wind would likely be much greater than the vertical drift caused by the magnus effect.

If I recall correctly, this myth honestly comes from civil war times. Soldiers would aim their cannons directly at the front line of soldiers coming at them, but the ball would "sail" over the first line and take out men in the second or third...which I would be prone to believing was an effect of their design, rather than the magnus effect.

Nomercy

Joe D., Read "Sniper" by Adrian Gilbert, I want to say chapter 7, but I'm not sure....bullet "Rise" in hot weather does honestly occur, but it isn't the bullet that's rising, it's the image of your target. It's called known as a mirage, and pretty accurately so. The hazy lines you notice on the roadway occur over all sorts of land, just to a lesser degree. The heated, rising air bends light, so what you're seeing isn't exactly what you're looking at.

Mirage can also be used to calculate wind speed, adjust your scope slightly out of focus and you can clearly see the mirage "lines" or waves, in a good wind, they'll be leaning one way or the other, I can't recall right now where exactly you're supposed to aim relative to the lines though? It's not even with the bottom, it seems like there is a rule of 1/4 in there somewhere?

Read military sniper manuals or shooting books written by retired snipers and you'll find a wealth of information.

Mirage and magnus effect are minimal, most hunters will never notice them in their lifetime, because the change in relation of POI and POA over typical hunting ranges isn't enough to take your bullet completely off your deer. Most guys will attribute a miss at that range to their "accuracy" or the wind, not to the smaller ambient environmental effects.

driftrider

Nomercy,

Then the real question is this...does the magnus effect due to a strong crosswind coming from the correct direction create enough lifting force to balance or overcome the force of gravity, under REALISTIC shooting conditions?

If it does, then a bullet could in fact experience enough body lift to actually climb above the plane of the barrel. If not, one could say that the force simply acts to retard the decent of the projective by cancelling out some of the accelleration created by gravity (like a parachute).

Mike

Bilge

Here's a little scenario I love to watch people try and wrap their brains around. If they get it, and it usually takes a little extra expanation, then they are in the right frame of mind to get other aspects of ballistics.

You have two bullets, one that you are going to drop by hand and one that will be fired from a gun. The gun's barrel is perfectly level with a perfectly level floor that extends for miles. If you drop your bullet from the same height of the gun barrel at the exact moment the fired bullet leaves the barrel then both bullets will hit the floor at the same time. (See attatched picture)

It's really cool seeing the "lights come on" when someone gets it.

DM

Another thing, that i've SEEN happen is, ammo fired in HOT weather, the powder makes more pressure (pressure = fps) and so shoots slightly flatter. Shoot the same ammo in very cold weather and the V is slower!!

I've seen this many times with 22lr ammo, when winter hunting in cold weather high in the mountains for ptarmigan!!!

Anyway, magnus theory or not, i'm sticking with, bullets start to fall from the time they leave the bbl, and keep on falling!!!

Drilling Man

Bilge

I agree driftrider. Considering that the viscosity of atmosphere is so minute compared to any liquid. And even in hurricane force winds (which I hope no one will be shooting at a deer then) and the rate of rotation is still only 1 in 10 inches for most rifles (fast time true, but not likely fast enough) and considering the tiny amount of surface area involved compounded by the density of the bullet (it's going to take a lot of lift to noticeably affect something made of lead and copper) that the Magnus effect would probably so small that it would only retard the rate of decent and even then you'd probably need to borrow a lab and a few NASA techs to separate the difference from standard deviation. Here's the real kicker though, using the old .30-06 as an example, the bullet is not just spinning in space it leaves the barrel at 1,984 MPH and if shooting in a 100 MPH crosswind the net effect would be a 0.05% total wind direction change over the surface of the projectile from shooting in calm conditions. That's about 9 degrees from head on and that would definitely not be enough, in my opinion, to generate the Magnus effect in any measurable quantity. Now you're no longer talking about the rotation being perpendicular to the air current, it's nearly parallel to the air current. Keep in mind that's with 100 MPH crosswinds! That's my opinion. But hey, it's been a long time since I was at Purdue and Nomercy, I'd love it if you'd run the numbers and prove me wrong (not with out debate of course ) because I want above all else to learn. In fact if you can demonstrate this does indeed have a quantifiable and meaningful impact on ballistics it will certainly only enrich my knowlege as well as others who read this board and after all that's why most of us are here.

I still believe that the bullet NEVER rises above the plane of the barrel! By the way, every physics model used by "ballistics software" I've tried out agrees with me on that one. If you plot bullet drop you wont find the bullet rising to a positive number at any point. Who knows maybe they "don't of what they speak" either (to paraphrase your post), in which case you could set us all straight.

Nomercy

I agree that any given bullet fired on any given day won't likely see much of a magnus effect...and even if it does, it is very unlikely that it actually rises at any point, as opposed to simply resisting its decent. However, I can say that it is feasible, with the working knowledge I have, that it could occasionally happen in the right conditions that are possible under "natural" and real hunting conditions.

One thing you are mistaken on, the magnus effect has absolutely nothing to do with linear velocity, i.e. the speed of the bullet. It ONLY has to do with the fluid velocity (wind speed) and the rate of rotation or angular velocity...a 3000fps bullet of any diameter coming out of a 1turn/12" bbl is spinning at 180,000rpm, which is pretty quick. See below for details.

This is about to get REALLY long.

Want proof the magnus effect is real? Why do curveballs curve? Check out a golfball when you hit it some time, ever noticed how sometimes they'll climb very sharply if you hit them with a lot of spin?

Ok, if you want numbers, here goes: a 150grn .30-06 bullet at 3000fps from a 1/12" rifle in a 15mph crosswind in air at 40Deg. F:

L=lift exerted on a rotating cylinder PER length

L=pGV, where p=density of fluid (air), V=velocity of fluid (or , G=2*(pi)*radius*v, v=2*(pi)*radius*w, w is angular velocity=linear velocity*rate of twist


SO: L=pGV-->L=p*2*pi*radius*2*pi*radius*linear velocity*rate of twist*wind speed

Throw in a couple conversions, like the radius in inches-->ft, wind speed in mph-->ft/s, and pound(mass) must be converted to pound(force)....this conversion is 1lbf=32.174lbm*ft/second squared.....

Plug in all the numbers and you get 1.07lbf/ft.

I'm going to assign a value now for the average cylindricity of a 150grn .308" bullet. If you assign an average radius of .154" (1/2 of .308"), your spitzer type 3/4" long bullet is likely now only .5" long...so I'll use that length for my effective force length....that's 0.042ft in case you're wondering...

That comes up to 0.045lbf force exerted upwards due to magnus effect (or downward if the wind were switched).

The force of gravity can be calculated Force=mass*acceleration-->150grns*1lbm/7000grns*32.17ft/second squared/32.17lbm*ft/lbf*second squared

That gives a force of gravity of 0.021lbf. Roughly half of that exerted by the magnus forces.

However, this is for a constant 15mph wind, and assumes idealized boundary layer separation (i.e. it doesn't involve boundary layer/vortex shedding), and it doesn't include the boundary layer buffering effect that the linear velocity of the bullet causes (the bullet rushing through the air reduces the magnus moment: the boundary layer is kind of pushed off the back of the bullet). Nor does it account for likely a hundred other effects that algebraic models (which i used for simplicity) can not account for, differential and integral values for the vortex shedding and linear velocity terms.

Do a search on the web for "magnus effect" or magnus moment, and you'll find a wealth of information on the topic.

Here's a website I found interesting, but you'd have to have access to a LOT of constants to use this formula http://www.snipersparadise.com/wound...m#MagnusMoment

Like I said, it's not likely, but experimental and theoretical models have proven the validity of the case.