What causes bullet drop?

[hartcreek]

I ran into some chuckle head that did not think that I should be sighting in my rifle in a particular area. This area had been open for muzzle loading deer and elk and the road hunters had chased everthing out of the area. I was muzzle hunting the last elk a few day before when the last cow and calf were run out and all fresh tracks were straight up the hillside. I live five miles and 2000 feet in elevation below.

In my area the local town shooting range is at an elevation of 700 feet. I plan on hunting with my 30-06 above 5000 feet and at a distance up to 600 yards. At that distance the difference in g between 700 feet and 5000 feet translates to a miss at 600 yards.

[JohnKSa]

At that distance the difference in g between 700 feet and 5000 feet translates to a miss at 600 yards.

Are you saying that the altitude difference will result in a miss, or are you separating the effect of air pressure difference and the effect due to the difference in g and saying that the difference in g alone will cause a miss?

What's the difference in point of impact due to change in g in that scenario?

[Ridgerunner665]

Gravity causes it to drop...the rate at which it drops is the same regardless of BC.

BC only appears to make them drop slower...because the other factor in "drop" is "time".

Higher BC retains velocity longer...so time of flight is less, so it doesn't have time to drop as much as a lower BC bullet.

The simple answer to your question though is gravity.

May have already been posted...I didn't read it all...just a fly by.

[Snyper]

What's the difference in point of impact due to change in g in that scenario?

My ballistics program shows a roughly 6" difference in the load I used going from 700 to 7000 ft, with less drop at higher alititudes.

I suspect it's due more to thinner air than any huge gravity differences, since other things besides height can cause gravitational anomalies

[HiBC]

I'll agree,relative to the line of the bore,drop begins immediately.

I was looking at it a different way,and it has to do with the advantages of putting some trajectory above the line of sight.The bullet is typically launched with some degree of upward toss before it falls.Different perspective.

Nothing worth a big discussion,though.

IMO,the folks trying to factor in gravity....I do not think it will be significant to anyone shooting a rifle.In any case,Iam shooting from the surface of the earth,I have the earth's mass touching my body at sea level or 10,000 ft.I guarantee it will feel like there is more gravity walking at 10,000 ft.

Maybe if I were flying at 60,0000 ft and shooting long range...

But the air getting thinner at altitude definitely has an effect at longer hunting ranges.Sight in at 2500 ft,then go on an 8500 ft hunt,yes,it may cause you a bad hit,or a miss.

[JohnKSa]

I was looking at it a different way,and it has to do with the advantages of putting some trajectory above the line of sight.The bullet is typically launched with some degree of upward toss before it falls.Different perspective.

That's precisely why this topic can be so confusing. There are at least three perspectives from which to view the problem. All of them correct and all of them quite different.

My ballistics program shows a roughly 6" difference in the load I used going from 700 to 7000 ft, with less drop at higher alititudes.

I suspect it's due more to thinner air than any huge gravity differences, since other things besides height can cause gravitational anomalies.

I agree. I would expect a significant change in drop (less drop) from the reduced air density from an increase in 6300ft altitude. My back of the envelope says that if you had a bullet with a BC of 0.502 at 500 feet, the effective BC of that same bullet at the same muzzle velocity and ambient temperature would be 0.639 at 7000 feet. That's a 26% improvement in BC--pretty significant.

On the other hand, the difference in gravitational force between sea level and 7,000 feet should be a little less than 0.07% if I've done my math right. Just to get a 10% drop in g, requires Space Shuttle type altitudes.

[rkittine]

Any of you golfers out there that have played golf at sea level and then a 7 or 8 thousand feet out west can attest to the difference that altitude has on flying objects.

All those things mentioned (not in a vaccum, bullet DC, twist and stabilization in the barrel) will effect beyond Newtown's Law.

Like a Bench Rest Shooter, keep a log of your shots and conditions and learn how things effect your gun and your loads.

Bob

[sirgilligan]

Some of the things that will affect the trajectory of a bullet:

Muzzle Velocity - time of flight
Ballistic Coefficient - drag, time of flight
Altitude - air density, powder burn rate
Humidity - air density, powder burn rate
Temperature - air density, powder burn rate
Barometric pressure - air density, powder burn rate

There are counter-intuitive things that happen as well. Higher altitude means thinner air, but dry air is thicker, so if you are at a high elevation but the air is extremely dry the two variables work against each other. Also, colder air is more dense than warmer air, and often with the altitude increase the temperature drops. Yes, humid air, air with more water vapor, is thinner.

Temperature affects the barrel as well.

I recommend getting a feel for how your gun and cartridge behave in the conditions you are hunting. A log book of the results is important to refresh your memory, but you can hardly go hunting one year and the next year find the same weather conditions the next year. That is why I feel that if you are serious about filling your hunting tag you use a cartridge that appears to be less affected by conditions, thus shooting a 300 Win Mag over a 243, stuff like that. Just my opinion. The more variation in conditions the more you need a fast, heavy bullet, with a high ballistic coefficient.

And we haven't even talked about wind drift, and something that few consider is that wind doesn't flow horizontal to the ground, there are down drafts, up drafts, and all the variations in between. Imagine shooting across a ravine, the shot may be 300 yards or so, but the air moving in that ravine can be going everywhere. Another argument for a fast heavy low bc projectile. The less time you are in the air, and the more mass you have, the less you will be pushed and pulled and slowed due to drag.

http://www.theweatherprediction.com/habyhints/260/

http://www.riflebarrels.com/articles...e_humidity.htm

http://www.accurateshooter.com/techn...-air-pressure/

http://longrangeshooter.com/2009/02/...fects-on-zero/

https://www.ballisticproducts.com/bp...coldpowder.htm

[natman]

I mean what factors will cause bullet drop to be greater for one load than another.

Drop for every bullet is roughly the same per second.

What causes trajectories to vary is a difference in velocity, which results in a larger horizontal distance per second. The faster the load, the farther away the bullet will be after one second's worth of drop, hence the flatter the trajectory curve.

What keeps the curve flat is a good Ballistic Coefficient, which means the bullets loses less velocity as it travels.

[hammie]

@JohnSKa in post #31:

I'm late to this party, but you did your math right. I used the same formula you did. The decrease in the gravitational constant with altitude decreases by the factor: ((Re)/(Re+h))exp2, where Re = earth mean radius (3959 miles) and h = altitude. I used an elevation change of 5280 ft, just to make the math easier. At 5280 ft, I got a factor for the "g" decrease change of .0005, which compares completely with your calculation of .0007 for 7000 ft.

Assuming that decrease in "g" is the only factor, then a drop of 12 inches at sea level would change to a drop of 11.994 inches at one mile elevation due to the change in the gravitational constant. When someone builds a rifle which will group at 6 thousands of an inch, then I will start worrying about the change in "g" with altitude at typical hunting elevations.

[tobnpr]

OP,
Not to complicate things for you, but keep in mind that bullet drop is also affected when shooting on a steep incline...

Up, or down, the bullet's flight depends on the horizontal distance to the target- not line of sight. Because this is due to the effect of gravity (drop) on the bullet, it must be compensated for on steep inclines.

A 400 yard line of sight shot on a 40 degree incline would be ranged as only 335 yards- a significant effect if shooting mountain goats

[rkittine]

And if you shoot straight down is the bullet drop 100% or 0%? For sure angle has a big influence on the effective drop.

Bob

[TimSr]

Gravity is a constant
Velocity is a variable


How fast it loses velocity (aerodynamics) is a variable

[Bella]

I thought air resistance, drag if you will, was a bigger factor than gravity.

[tahunua001]

All objects, even bullets, drop at the same rate regardless of weight.

please cite a scientific text for this information. that is simply not true. as it has been said already, a simple egg toss performed by most elementary school students is enough to disprove your statement. drop an egg, pingpong ball, and lead weight from the top of a set of bleachers and time how long they take to hit the ground, the most dense object hits the ground first as the three have virtually identical ballistic coefficients. there is a gravitational constant, but air resistance and mass also have to come into play. dust can remain airborne for several minutes, even hours, yet it has the same density as the rock that spawned it. this is because the mass of the object added to the gravitational constant is negated by atmospheric resistance. only in a vacuum do all objects fall at the same rate regardless of weight.

[MrBorland]

Originally Posted by tahunua001

only in a vacuum do all objects fall at the same rate regardless of weight.

This was already mentioned. Besides, for the purposes of the OP's question, though, "all objects fall at the same rate" is a pretty good 1st approximation, as it explains the general concept of relative bullet drop without getting too complicated.

[natman]

I thought air resistance, drag if you will, was a bigger factor than gravity.

No.

If there were no gravity the bullet wouldn't drop at all. If somehow there were air but no gravity the bullet would travel in a straight line until it finally stopped, floating in air.

If there were gravity but no air the bullet would drop but would still be going at muzzle velocity when it hit the ground.

Gravity. It's not just a good idea, it's the law.

[mehavey]

Back to the basic OP question.

Notwithstanding bullets made of feathers, all bullets drop at the same rate* (rate being measured in time)
All bullets fired out of a level rifle sitting on a bench three off the ground will hit that ground in about 4/10th of a second.
Period.

If one wants to measure in terms of drop per distance traveled, that's a different problem.
How far they travel in through that fluid called air (or water, or molasses, or whatever, etc), within that 4/10ths second
is a function how quickly they slow down -- streamline bullet shape & bullet weight -- or drag vs intertia.





* Yeah, I know... not the same rate but rather the same acceleration.
...But Hey ! What's another "/t" between friends ?

[MrBorland]

Originally Posted by natman

No.

Again, for the purposes of the OP's original question, I think it's safe to assume gravity's a constant. Yes, gravity is what's acting on the bullet, but a bullet's ability to maintain it's velocity (i.e. it's resistance to air resistance) is what explains relative bullet drop.

This threads starting to get repetitive and into details that only muddy an answer that's already been made clear, IMO.

[hammie]

@Borland & mehavey: In my high school physics class, the teacher had a device which shot a marble horizontally while simultaneously dropping another marble vertically. While one marble traveled about 20 feet away, they both clattered to the floor at the same time.

The teacher also had a device which would sit inside a large bell jar and which would drop a marble and a feather at the same time. After evacuating the jar, the marble and feather would hit the bottom at the same instant. That demonstration really stuck in your mind.

Is physics even taught in high school any more? I guess it's mainly feelings, sensitivity and social justice now. But that's a different topic for another thread in a different forum.

[Bushmaster1313]

As soon as bullet clears the muzzle the earth begins to rise towards the centerline of the barrel.

Happens every time.

[44 AMP]

dust can remain airborne for several minutes, even hours, yet it has the same density as the rock that spawned it.

Yes, and ..no.

Each tiny little speck of rock dust is the same as the rock it came off of, BUT density is a concentration per unit volume. A cubic centimeter of rock and the same volume of rock dust in air are not the same density, despite each particle of dust being the same as the rock. IT is a mixure (dust particles and air), and has a different makeup, and density than the solid rock.

The pull of gravity is constant, (at a given distance from the center of the earth), so the pull on objects is the same. All try to fall at the same speed, because they are being pulled down by gravity at the same amount.

It is the relationship between surface area of the object, its weight and the resistance of the air it must pass through that changes the velocity different objects attain as they fall. This is where "terminal velocity" comes in.

Drop a 200lb man and a 200lb lead brick from high enough, and each, as they fall through the air, will eventually reach different terminal velocities. Both are under the same "pull", the acceleration due to gravity, but the air resistance to the falling man's greater surface area will eventually reach a balance with the pull of gravity, and the body will not fall any faster, no matter how much further it falls.

The same thing happens to the lead brick, but because the surface area is many times smaller than the man, the brick will accelerate longer before air resistance balances the acceleration due to gravity. Much longer, actually. SO the brick is moving much faster by the time it reaches its terminal velocity.

It may just be semantics, or your point of view. Mine is that everything falls at the same rate of acceleration speed (the pull of gravity), but it is a combination of weight, surface area, air resistance, and time that determines the final velocity a falling object reaches (on earth).

[mehavey]

Originally Posted by hammie

...teacher also ...drop[ed] a marble and a feather at the same time [in a bell jar].
After evacuating [the air]..., the marble and feather would hit the bottom at
the same instant.

Is physics even taught in high school any more? I guess it's mainly feelings,
sensitivity and social justice now.

Social Justice today says you take away everybody's air.

And now back to our regular programming....

[hammie]

This is getting pretty deep. Density is mass per unit volume, and assuming that the rock is homogeneous (does not refer to rock's gender preference), any particles from the rock would have the same density as the rock. An air/rock particle mixture would have a different density, but I can make the original rock equivalent to that density, by just addng a large enough air envelope around it.

And to make things more complex, not all particles accelerate to a terminal velocity. They may be small enough that they won't fall at all. Plug colloidal dispersions or "Brownian motion" into your search engine (and no, Brownian motion has nothing to do with John Moses Browning).

[jmr40]

My ballistics program shows a roughly 6" difference in the load I used going from 700 to 7000 ft, with less drop at higher alititudes.

Sounds about right, but is only relevant if your rifle shoots 1" groups at 600 yards. For most of us if we fired 10 shots at a 600 yard target at 700' elevation, then used the same target and fired 10 more shots at 7000' elevation, you'd have a hard time seeing 2 distinct groups. Some shots would overlap on the target.

At 200-300 yards, which is about the max range for most hunters the difference is not enough to worry about. For guys who target shoot, or hunters who practice long range shooting it is not an obstacle that cannot be overcome.