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Old October 18, 2014, 06:09 AM   #1
Pond, James Pond
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What causes bullet drop?

Don't say Gravity.
Let's be sensible and take that as a given.

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

My initial guesses are, from the same gun & calibre (eg .308), velocity and bullet weight and BC.

True? And, if so, what else?
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Old October 18, 2014, 06:34 AM   #2
RaySendero
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Bullet Drop

Your question is so basic, I suspect there's another question coming.

Well anyway,
I've seen wind cause both drop and rise shooting 22LR targets.
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Old October 18, 2014, 07:11 AM   #3
Pond, James Pond
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Your question is so basic, I suspect there's another question coming.
Not really.

A bullet starts to drop the second it leaves the barrel, but even at the same speed not all bullets will drop at the same rate, nor along the same trajectory.

I want to know if there are factors beyond those listed in the OP, that would have noteworthy effect on this....
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Old October 18, 2014, 07:21 AM   #4
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Ballistics

In theory, if you were to drop an identical projectile from the same exact height and at the same exact time that the same projectile left the barrel of gun perfectly horizontal firearm, they would hit the ground at the same time. Old Newton here. Therefore based on the velocity that the projectile leaves the barrel that would be the single MOST critical criteria for the bullet drop. In theory the actual thing you drop would not even have to be equal in any way, but of course wind resistance for a feather is a lot greater than for a hunt of lead.

In real live things like wind resistance, sectional density, stability of the projectile (rifling rates etc.) certainly have an effect. Loss of mussel energy too when shooting longer distances for a given round.

For those that have a chronograph or access to one, set up on a bench and check the velocity close to the barrel, Then place the chronograph in front of your target at the distances that you normally shoot and shoot and see what the velocity is there. It will tell you a lot of expected performance at different ranges.

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Old October 18, 2014, 07:23 AM   #5
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Bullet drop can be easily explained.
It's caused by our spinning and rotating planet and Newton's laws of physics.
Next question...
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Old October 18, 2014, 07:28 AM   #6
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Quote:
Originally Posted by Pond, James Pond
at the same speed not all bullets will drop at the same rate, nor along the same trajectory.
All object "drop" at the same rate. How far they drop is dependent on the time they're allowed to drop. Bullets with lower BCs shed velocity quicker, so even if they have the same MV, they shed forward velocity faster than a bullet with a high BC. Generally, within a caliber, lighter bullets have lower BCs, so they shed forward velocity quicker than a heavier bullet. Their time of flight is therefore longer, and since they're allowed to drop longer, they drop farther.
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Old October 18, 2014, 08:07 AM   #7
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Quote:
In theory, if you were to drop an identical projectile from the same exact height and at the same exact time that the same projectile left the barrel of gun perfectly horizontal firearm, they would hit the ground at the same time. Old Newton here. Therefore based on the velocity that the projectile leaves the barrel that would be the single MOST critical criteria for the bullet drop. In theory the actual thing you drop would not even have to be equal in any way, but of course wind resistance for a feather is a lot greater than for a hunt of lead.
Quote:
All object "drop" at the same rate. How far they drop is dependent on the time they're allowed to drop. Bullets with lower BCs shed velocity quicker, so even if they have the same MV, they shed forward velocity faster than a bullet with a high BC. Generally, within a caliber, lighter bullets have lower BCs, so they shed forward velocity quicker than a heavier bullet. Their time of flight is therefore longer, and since they're allowed to drop longer, they drop farther.
Nice answers!! Much clearer.
Thanks!
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Old October 18, 2014, 10:53 AM   #8
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Yeah, I was under the impression all bullets drop at the same rate, the difference is velocity. A faster bullet fired at the same angle will travel farther before it hits the ground. BC is only a factor because it effects how quickly a projectile will slow down due to air resistance.
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Old October 18, 2014, 11:01 AM   #9
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Quote:
All object "drop" at the same rate. How far they drop is dependent on the time they're allowed to drop. Bullets with lower BCs shed velocity quicker, so even if they have the same MV, they shed forward velocity faster than a bullet with a high BC. Generally, within a caliber, lighter bullets have lower BCs, so they shed forward velocity quicker than a heavier bullet. Their time of flight is therefore longer, and since they're allowed to drop longer, they drop farther.
That is it. I wrote the ballistics calculator for my Gun Log apps and had to learn about ballistics. I read a lot of papers, had to get out my college physics books and re-learn what I had forgot from college.

The amount of time an object is falling is directly related to how far it will fall. If you drop a basketball from your chest it will fall to the floor in less than a second. If you climb to the top of the empire state building and drop the basketball it will take longer than a second to hit the ground. The longer something is falling the further it falls.

If I shoot a bullet at 3000 fps with a ballistics coefficient (BC) of 0.5 it will take it .33 seconds to arrive at a target 300 yards way. If the bullet it only travelling 2800 fps it take is .36 seconds to get to 300 yards, that is 3 hundredths of a second longer. During that extra 300 hundredths of a second that is nearly 3" of more drop.

Maintaining speed is what the ballistics coefficient is all about. A bullet that cuts through the air better slows down less and maintains its speed, thus reducing the time to get to the target and thus dropping less because it was falling for less time.
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Old October 18, 2014, 12:00 PM   #10
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All objects drop at the same rate in a vacuum.

In air, the objects drop at a rate determined by the interaction of gravity and the air resistance. Very dense objects (rocks, bullets) fall relatively fast, objects that aren't dense (feathers, Ping-Pong balls) fall more slowly.

If you have two bullets of the same size/shape, the one that is more dense will fall a tiny bit faster. HOWEVER, the difference isn't really significant since bullets are all pretty dense, they're all shaped about the same (in terms of the profile they present to the ground) and they usually aren't falling a long distance.

In practice, the only significant contributor to bullet drop is time of flight. If you have two identical bullets, the one that takes the longest to get to the target will drop the most. That's because gravity has more time to act on it. sirgilligan has it right.
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Old October 18, 2014, 12:01 PM   #11
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James,

The answer to why and how fast bullets drop is simple - gravity.

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

With a bullet, the question may more precisely, how much drop will a bullet of a given weight and velocity drop over n yards'

The drop rate is identical with all bullets since they never achieve their terminal velocity in a downward direction unless they are shot straight up.
Then they achieve their terminal velocity on the way down.
However, a bullet traveling faster than another bullet towards a target will be covering the distance to the target in less time so it will be dropping less time before it hits the target. Therefore, it will have dropped less distance before it hits the target, so it appears that it drops less. The time it is in the air determines the amount of drop.

That is why there are "flat shooting bullets" like a .22-250 40 grain bullet that leaves the muzzle at over 4000 fps and bullets that shoot in big arcs like a 22LR that is travelling at 1100 fps out of the muzzle.
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Old October 18, 2014, 12:05 PM   #12
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Just to add to the conversation, Ballstic Coefficient is a proxy for "drag."

A higher BC means less drag at any given velocity verses a bullet with a lower BC. The mass of the projectile actually has something to do with it as mass is resistance to force (force = mass*acceleration, therefore Force/Acceleration=mass).

Since drag is an acceleration dependent on velocity, more mass means more resistance to drag (generally expressed as more momentum).

But, if two bullets have the same BC and different masses, the lighter one will have less drop if it can be shot faster.

Hence the conventional wisdom of long range shooters, shoot the highest BC the fastest you accurately can.

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Old October 18, 2014, 12:40 PM   #13
Pond, James Pond
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OK, so it really was that simply. Thanks for explaining, folks!

There is only one thing I have in common with sirgilligan: we had both completely forgot high school physics.. What separates us is that I didn't re-learn it!!

So really it now comes dow to velocity and drag that dictates where a bullet is at a given distance from the muzzle.
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Old October 18, 2014, 01:06 PM   #14
Dragline45
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Quote:
Don't say Gravity.
Let's be sensible and take that as a given.
Saying gravity would be sensible
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Old October 18, 2014, 01:15 PM   #15
Pond, James Pond
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Saying gravity would be sensible
Perhaps, then. But would it be helpful?

What I did not know/remember was that the same thing drops at the same rate, but where depends on speed and drag and yet another day has passed when I've learnt something thanks to TFL.

TFL is good.

Pond likes TFL.
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Old October 18, 2014, 01:17 PM   #16
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Mr. Pond,

Certain barrel conditions, a damaged grown for instance can cause the bullet to exit the barrel, in different orientations. It could be in any direction, depending upon where the crown is defective. If the defect causes deviation negative in the Y axis it could appear as greater drop, but gravity still would have the far greater effect.
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Old October 18, 2014, 04:18 PM   #17
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You guys have it basically covered except you forgot that there can be variations in g. If I sight in my rifle down at my folks place and then go hunting on a long range shot I may well miss that deer because of the difference in g. The atmospheric pressure is less at higher elevations. You need to sight in at the elevation that you are going to hunt or shoot.
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Old October 18, 2014, 05:17 PM   #18
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Quote:
I may well miss that deer because of the difference in g.
I've used that excuse for missing, as well as the "earthquake just as I was squeezing the trigger"

It would take a huge variation in gravity to make any measurable difference in the point of impact

Bullets fired LEVEL, from the same height, all have the same amount of drop.

The only difference is in the HORIZONTAL distance traveled before they hit the ground

But no one fires them on a level plane, so then it starts to get more complicated
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Old October 18, 2014, 06:08 PM   #19
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You guys have it basically covered except you forgot that there can be variations in g. If I sight in my rifle down at my folks place and then go hunting on a long range shot I may well miss that deer because of the difference in g.
You may miss the shot but it is unlikely that the miss would be because of differences in g. Those differences are small compared to the other trajectory changes induced by the different altitude. You would most likely miss because the thinner air presents less drag and the bullet doesn't slow down as fast. That would result in a shorter time of flight and therefore in less bullet drop.
Quote:
The atmospheric pressure is less at higher elevations. You need to sight in at the elevation that you are going to hunt or shoot.
This is correct. The difference in trajectory due to elevation changes is primarily due to the difference in air pressure (air density) which changes with altitude.
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Old October 18, 2014, 06:11 PM   #20
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To confuse the issue just a bit:The drop begins from a presumed level barrel.We don't usually do that.As the bore is usually inclined,to place the bullet on target at zero 'd range,in effect the bullet is tossed up ,till it achieves maximum elevation at midrangetrajectory.("I sighted in 3.5 in high at 100 yds to put me on at 300)

So,actually drop does not begin until the time and distance have passed to put the bullet at the peak of trajectory.For a 300 yd zero,this may be at about 200 yds.At this point,drag has slowed the bullet,and the bullet will begin its drop from above the line of sight,by the height of the midrange trajectory.

We don't just drop the bullet,we give it a little toss up first.
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Old October 18, 2014, 06:26 PM   #21
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Not really. The bullet still begins to drop the instant it leaves the barrel (what else would keep it up?). It may "rise" in relation to the ground, but it still drops in relation to the line of the barrel.

Jim
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Old October 18, 2014, 07:01 PM   #22
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Correct. The bullet never rises with respect to the line of the bore, it only drops from the line of the bore.

If the barrel is aimed upwards (above horizontal) then the bullet rises with respect to horizontal (at least in the early part of the trajectory) but still not with respect to the line of the bore.

Said another way, if you attached a laser to the top of the barrel of a gun so that it is parallel to the bore but just a hair above it, a bullet fired from that barrel would never be illuminated by the laser at any point in its flight. That's true even if the bore were to be pointed upwards.

To confuse the issue a little more, most of us instinctively think of bullet travel in terms of the line of sight. The line of sight is NEVER the same as the line of the bore. If it were, we could never hit anything unless it was below the line of the bore of the gun. The line of the sight is angled DOWN with respect to the bore. So when we look through the sights at the target, the bore is angled UPWARD with respect to the line of the sights.

That makes the bullet initially travel upwards (with respect to the line of sight) and so we tend to think that the bullet has risen. It HAS risen in relation to the line of sight, but NOT with respect to the line of the bore.
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Old October 18, 2014, 08:29 PM   #23
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Yes, the gravitational constant is different at different altitudes. In my ballistics calculator I do correct "gravity" for altitude, but I only do it at the beginning of the calculation because typically we are looking at bullet drops in inches and maybe up to a yard or so for typical shooting.

Currently the calculator uses a stepping algorithm.
The bullet starts with a certain muzzle velocity, ballistic coefficient, sight height and bore axis.
Then I calculate where how long it takes the bullet to travel an inch. That time is time of flight. With that time I can calculate how far it dropped. I also calculate how much the bullet slowed down due to drag. Now the bullet is moving slower and I calculate how long it now takes it to travel an inch. The process repeats over and over, recalculating each step.

If I wanted I could recalculate gravity for each step as well, but it doesn't really matter for such small changes in height above the center of the earth.

Falling objects
http://youtu.be/zMF4CD7i3hg

This one is really fun:
http://youtu.be/D9wQVIEdKh8

Objects in a vacuum
http://youtu.be/AV-qyDnZx0A

And here is the one I remember from College, my favorite:
http://youtu.be/cxvsHNRXLjw
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Old October 18, 2014, 08:47 PM   #24
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"The line of the sight is angled DOWN with respect to the bore."

Not always. Look at, say, an S&W revolver, and you will see that when the bore is horizontal, the line of sight is up. Put another way, if the line of sight is horizontal (shooter is aiming at a point which is at the same height as his eye), the barrel will be pointing down. I'll let folks figure out why.

Jim
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Old October 18, 2014, 09:31 PM   #25
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Correct. In some handguns, recoil lifts the muzzle enough between the time of the trigger pull and the time that the bullet leaves the muzzle that the sights have to compensate for that movement. In that situation the line of sight at the time of the trigger pull is angled up with respect to the bore or may be roughly parallel with the bore.

To include that situation in this discussion, my original statement should be amended to say:

The line of sight at the instant that the shooter pulls the trigger is angled DOWN with respect to line of the bore at the moment that the bullet leaves the muzzle.
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