What causes bullet drop?

[Snyper]

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.

It's relevant since the question I replied to was the difference in drop at 600 yds

Group size makes no difference since "drop" would be figured from the center of the group, and not from individual shots

In theory, a gun that shoots a 1" group at 100 is capable of a 6" group at 600, but the centers of the groups can still be at different elevations on the target, even if the edges overlapped

[pathdoc]

For all rifles, projectile drop is determined by gravity. Distance = 16 x the square of the time in seconds in a vacuum, but drag in the vertical direction is almost vanishingly small compared to the drag in the direction of line of flight.

What distance a projectile has dropped at a specific range is determined by time of flight to that range. The fact that velocity is always decreasing with range complicates the calculation hideously.

Velocity remaining at, and time of flight to, a given range are dependent on drag, which is dependent on air density (pressure and temperature) and ballistic coefficient, which is in turn dependent on the sectional density and the form factor (shape, basically) of the bullet. (It's also dependent upon velocity, which is why you either need a mastery of calculus or formulae which make good approximations and can make the calculus go away).

All bullets will descend (in the vertical plane) at basically the same rate, because the vertical rate of descent is so slow, the "side-on shape" of the bullet makes little difference.

All bullets begin an immediate descent under gravity from the line of bore from the moment they leave the muzzle, and never rise above it. They DO rise above the line of sight, by a degree determined by angle of elevation of the rifle barrel (determined in turn by the range you have sighted in for). Then of course they descend, meet line of sight once more, and begin to drop below it.

The more you read, the more you delve into this, the more hideously complicated it gets. And don't get me started on wind, or on shooting at a significant up or down angle. That makes my head spin.

[Buzzard Bait]

Delay in flight

[Bart B.]

190's from my .308's need almost 5 MOA more elevation to zero at 1000 yards at 580' elevation than at 6600' with the same load and ambient temperature fired in the same barrel. There's been people who shot their rifles at the NRA 1000-yard range near Raton, NM, at 6600' elevation then used the same ammo and zeros at Camp Perry's 1000-yard line 580' above sea level and never got on paper. Their bullets hit the dirt below the target.

In my test with a 50 grain inflated balloon and a 50 grain bullet, both at ambient temperature dropped 10 feet, the bullet fell the fastest at 4980' altitude.

[tahunua001]

44 amp,
you essentially said in a few detailed paragraphs what I glossed over in a single sentence. thank you.

[Bart B.]

In both reality and common sense theory, a 1 inch group at 100 yards will open up at least 10% for each additional 100 yards of range. It's caused by the spreads of muzzle velocity, bullet BC (small spread caused by bullet unbalance) and air movement in the trajectory. And for each 100 yard range band, the percentage increases. For example, vertical shot stringing at 100 yards with a 50 fps muzzle velocity spread from a .308 Win is about 1/10th inch. At 600 yards, it's 4 to 5 inches. At 1000 yards, it's around 20 inches.

All this without bullet drop compensation. If bullets leave the muzzle on its axis upswing such that slower ones depart at higher angles, long range groups will be smaller in MOA than mid range ones.

[briandg]

James, every bullet, every object, no matter what, will drop at the rate of 32 feet per second, per second. (G) the only altering circumstances are air resistance to friction, and that is insignificant in bullets. A bullet fired downward will have a certain velocity imparted by the rifle from the moment it leaves the barrel, and it will still gain downward velocity at the rate of G/.

A rifle fired upwards will be accelerated upwards at the velocity imparted by the rifle, and downward pull at the rate of G will be pulling it downwards from the very second it leaves the barrel.
a bullet fired at an absolute level, directly perpendicular to the draw of gravity, will fall at absolutely the same rate of speed as a bullet dropped by hand. If a bullet was fired at well over a mile, at exact level, and a bullet was dropped at the very moment that the first bullet left the barrel, they would hypothetically arrive at the very same level when the bullet had traveled the full distance. This assumes absolute accuracy of the shot.

A fired bullet falls at the same rate as a dropped one.

Trajectory fluctuations are caused by only one thing. Changes of initial velocity, and the time it takes to reach target from the line of fire. A slow round drops more because it is accellerating downwards at G and it falls for a longer period of time than a super velocity round will.

Every bullet will drop 32 feet in exactly one second in earth gravity in vacuum. So will a bowling ball, a feather, or a space station.

[Pond, James Pond]

Nice explanation, Briandg.

Thanks.

And if you could do a Youtube clip of dropping a space-station in a vacuum alongside a Hornady Amax, I would be particularly grateful!

[Bart B.]

My info in post 54 has been discussed with many shooters. We all observe the same thing.

The denser air is, the more it slows down bullets.

[gman3]

Bullets "Fly" just like an airfoil on an aircraft wing flies. The shape of the bullet determines exactly how aerodynamic the projectile is. Because there is a lack of continuous propulsion, it becomes a ballistic object subject to initial propulsive force. The shape, speed, atmospheric conditions and aerodynamic efficiency determines the rate at which the force of gravity pulls it towards the center of the earth as it decelerates.

[briandg]

That is not correct. bullets do not skate on wall of air like an airfoil, they receive identical force from every angle of the bullet shaft. there is nothing that could cause a bullet to drift upward or downward because of friction interaction with air.

bullets drop at the prescribed rate of 32fpsXX. It's that simple. The same goes for wind drift. High velocity bullets with low drag spend less time with gravity acting on them, pulling them down, and they spend less time interacting with wind, and being pushed down, or sideways.

[JohnKSa]

...they receive identical force from every angle of the bullet shaft.

That is correct as long as they are flying point first and there are no significant air currents impinging on the bullet from the sides.

...the only altering circumstances are air resistance to friction, and that is insignificant in bullets.

That is generally a reasonably accurate statement for bullets that are falling in a normal fashion as long as they don't fall far enough to get close to reaching terminal velocity.

The statement needs to be qualified somewhat because air resistance does play a very significant role in terms of slowing down the bullet's velocity with respect to the bullet's travel along its aimed trajectory.

A bullet fired downward will have a certain velocity imparted by the rifle from the moment it leaves the barrel, and it will still gain downward velocity at the rate of G/.

There will be a downward force acting on the bullet due to gravity. Whether or not the bullet gains downward velocity will depend on whether or not that force is greater or less than the force created by air resistance.

My best estimate is that the terminal velocity of a stabilized rifle bullet travelling downwards point first is somewhere between 500fps and 1000fps. If the muzzle velocity is greater than the terminal velocity, the bullet will slow down in spite of the downward force of gravity. It will continue to slow down until terminal velocity is reached.

[SSA]

Every bullet will drop 32 feet in exactly one second in earth gravity in vacuum.

16 feet the first second. 48 feet the second second.

[Bart B.]

See how your ballistics software calculates drop.

"Shoot" different caliber bullets with different weights and shapes different speeds going out level then check the drop values at 1.00 second time of flight.

[Snyper]

See how your ballistics software calculates drop.

"Shoot" different caliber bullets with different weights and shapes different speeds going out level then check the drop values at 1.00 second time of flight.

What would be the point?

[Bart B.]

To see if they all give identical results for the same inputs.

And see if their results match what a hand-dropped bullet falls in one second's time.

I'd bet they don't.

[SSA]

I don't have a ballistic program that lets me input a level barrel with no zero range.

[Bart B.]

Enter sight height as zero inches, target range to 5 yards, range steps to 5 yards and max range to 1000 yards. Good enough for reasonable results.

[SSA]

No, that is not the same thing.

[Bart B.]

Here's the output from Berger's ballistic software for two bullets. Inputs are:

30 caliber 175-gr with G1 BC of .500
Zero sight height, 1 yard zero, 1 yard range step.
Max range 1000 yards and 70 deg. F.
Muzzle velocities 1600 and 2600 fps.

Drop after TOF of about 1.00065 second average as calculated for each muzzle velocity are:

1600 fps; TOF=1.0008 sec, range=446 yards, drop = 173.14" (14.43 feet) below LOS.

2600 fps; TOF=1.0005 sec, range=671 yards, drop = 164.19" (13.68 feet) below LOS.

[briandg]

16 feet the first second.

That's right. Too long since physics class and too tired to think. Skipped a step.

The rough part about this is that no matter how precise the measurements are and how good the program is, air density, humidity, wind, all sorts of things can get in the way of computer projected trajectory tables and send them off a few MOA.

Modern snipers absolutely blow my mind. How a man with a rifle manages to make hits on a 12" diameter target at a mile is beyond me. It's like magic.

[Stevie-Ray]

What if anything happens if I shoot my rifle at a range here near the 45th parallel, and then take it down to Ecuador to shoot it, where gravity is not as strong. Difference in time falling, farther shooting, etc.?

[JohnKSa]

where gravity is not as strong. Difference in time falling, farther shooting, etc.?

Probably not a measurable difference. While the force of gravity does vary depending on where you are on the earth and also with altitude, it doesn't vary as much as one might expect. As mentioned earlier in the thread, to get the force of gravity to decrease 10%, you need to be at orbital altitudes.

[Snyper]

Drop after TOF of about 1.00065 second average as calculated for each muzzle velocity are:

1600 fps; TOF=1.0008 sec, range=446 yards, drop = 173.14" (14.43 feet) below LOS.

2600 fps; TOF=1.0005 sec, range=671 yards, drop = 164.19" (13.68 feet) below LOS.

It's still pointless rambling

[Bart B.]

Only to those who cannot grasp its significance.