What causes bullet drop?
- Thread starter Pond James Pond
- Start date
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.
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.
sirgilligan
New member
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
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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"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
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
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.
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.
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.
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.
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?
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Ridgerunner665
New member
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.
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.
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'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.
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.
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.
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.
Altitude and Other Variables
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
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
New member
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/bullets_ballastics/ballistic_altitude_temperature_humidity.htm
http://www.accurateshooter.com/technical-articles/ballitics-altitude-and-air-pressure/
http://longrangeshooter.com/2009/02/temperature-effects-on-zero/
https://www.ballisticproducts.com/b.../curmudgeon_articles/060329_hotcoldpowder.htm
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/bullets_ballastics/ballistic_altitude_temperature_humidity.htm
http://www.accurateshooter.com/technical-articles/ballitics-altitude-and-air-pressure/
http://longrangeshooter.com/2009/02/temperature-effects-on-zero/
https://www.ballisticproducts.com/b.../curmudgeon_articles/060329_hotcoldpowder.htm
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.
@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.
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.
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
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
tahunua001
New member
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.