What causes bullet drop?
- Thread starter Pond James Pond
- Start date
briandg,
Don't feel bad. Sierra's external ballistics documentation disagrees with Bryan Litz about how bullets behave in the wind.
Sierra says the wind pushes the nose downwind, Litz says the wind shifts the net drag effect on the projectile to push the base downwind. Both modeling systems can accurately predict bullet impact in wind conditions, so functionally how it works doesn't really matter to the end user. I think Litz is right because his model also explains the bullets yawing over to align with the dominant air stream as they fall towards their target.
The real question is whether a crosswind imparts a perpendicular velocity to the bullet, or whether net drag stabilization will reorient the bullet parallel to original line to target after experiencing a gust. Benchresters have tested this at short ranges, and say that a gust at the firing line has more effect on the impact than a gust at the target line, which strongly implies that bullets once deflected will behave just like gyroscopes in that they will return close to original orientation, but not exactly.
Jimro
Don't feel bad. Sierra's external ballistics documentation disagrees with Bryan Litz about how bullets behave in the wind.
Sierra says the wind pushes the nose downwind, Litz says the wind shifts the net drag effect on the projectile to push the base downwind. Both modeling systems can accurately predict bullet impact in wind conditions, so functionally how it works doesn't really matter to the end user. I think Litz is right because his model also explains the bullets yawing over to align with the dominant air stream as they fall towards their target.
The real question is whether a crosswind imparts a perpendicular velocity to the bullet, or whether net drag stabilization will reorient the bullet parallel to original line to target after experiencing a gust. Benchresters have tested this at short ranges, and say that a gust at the firing line has more effect on the impact than a gust at the target line, which strongly implies that bullets once deflected will behave just like gyroscopes in that they will return close to original orientation, but not exactly.
Jimro
I asked such a (broadside) question after reading briandg's comment stating gyro stabilization keeps a bullet horizontal (if fired so) and it wouldn't nose down keeping its long axis parallel to its trajectory.
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Cross winds do cause vertical (perpendicular) movement to bullets. 22 rimfire ones string from 10 o'clock to 4 o'clock on target when the wind swings from the right to the left.
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On briandg's comment on hitting 12" areas at a mile, it's my belief that doing so with the first shot is 99+% luck. Here's what bullet dispersion will be on a target a mile away with a .300 Win Mag's 30" barrel:
1760 yard data for 30 caliber 220 gr Sierra HPMK leaving at 2900 fps
1049 fps on target
3.18 seconds tof
24" drift per crosswind mph
11" vertical spread per 10 fps of velocity change
15" vertical spread from small differences in BC due to bullet unbalance
16" drop for each 10 yards of range
I doubt any .300 Win Mag will shoot inside 20" that far away. To say nothing of how good the marksmanship, range estimating and wind doping qualities of the shooter are. There are "accuracy cones" for each the parts of the shooting system. When the limit for each one adds up in the same direction for a given shot, the bullet misses the intended target point by a long, long ways.
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Cross winds do cause vertical (perpendicular) movement to bullets. 22 rimfire ones string from 10 o'clock to 4 o'clock on target when the wind swings from the right to the left.
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On briandg's comment on hitting 12" areas at a mile, it's my belief that doing so with the first shot is 99+% luck. Here's what bullet dispersion will be on a target a mile away with a .300 Win Mag's 30" barrel:
1760 yard data for 30 caliber 220 gr Sierra HPMK leaving at 2900 fps
1049 fps on target
3.18 seconds tof
24" drift per crosswind mph
11" vertical spread per 10 fps of velocity change
15" vertical spread from small differences in BC due to bullet unbalance
16" drop for each 10 yards of range
I doubt any .300 Win Mag will shoot inside 20" that far away. To say nothing of how good the marksmanship, range estimating and wind doping qualities of the shooter are. There are "accuracy cones" for each the parts of the shooting system. When the limit for each one adds up in the same direction for a given shot, the bullet misses the intended target point by a long, long ways.
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That's a very good way of saying it. I've tried explaining that so many times. So your rifle can do 1.5 moa, the company guaranteed it, right? and so you feel perfectly confident shooting at a pronghorn at 400 yards, because that's a 6 inch guaranteed accuracy at 400?
Sure. Let's say your gun was bumped a bit during travel. Let's say that a skeeter hit the bullet. Let's say that you are tired and your eyes hurt, and you flinched a little, there was a bit of a draft about half way there, and you misjudged the distance by fifty yards. A shot fired that way could still hit the exact point of aim and nail that critter right in the heart, but it also may wind up five feet away because everything failed at once, in the same direction.
Sure, I can hit a tree rat with my .22 lr. All the way out to 100 yards. Unfortunately, that's a pretty small target, and there are a lot of things that can happen between the barrel and the brain. it's pretty likely that if I have just hiked a half mile, the air is breezy and damp, and I've had too much coffee, I'm going to count myself lucky if I manage to get within a couple of feet.
If a rifle's guaranteed to shoot 1.5 MOA at a hundred yards, it won't do that at 400. A group's subtended angle increases as range increases. Bullet BC and drop variables plus air movements and density in the trajectory cause it to get bigger. Most factory rifles with factory ammo will have their 100-yard groups open up 15% to 20% for each hundred yards past the first one. Which means if it shoots 1.5 MOA at 100 yards, it'll be abouit 2.0 to 2.5 MOA at 400.
Note some of those accuracy cones are bell shaped. Their diameter gets bigger as range increases. Included are muzzle velocity, atmospheric conditions and the bullet's BC. As the bullet slows down, the cone diameters get bigger.
Most interesting is the wind issue. A 1 MPH crosswind in the first third of the range causes about 3 times as much drift at target range than a 1 MPH crosswind in the last third. The wind's faster at the highest point of the trajectory than in the line of sight; it varies depending on terrain.
I don't think a bullet would hit point of aim zeroed at 400 yards with all the issues mentioned above.
Note some of those accuracy cones are bell shaped. Their diameter gets bigger as range increases. Included are muzzle velocity, atmospheric conditions and the bullet's BC. As the bullet slows down, the cone diameters get bigger.
Most interesting is the wind issue. A 1 MPH crosswind in the first third of the range causes about 3 times as much drift at target range than a 1 MPH crosswind in the last third. The wind's faster at the highest point of the trajectory than in the line of sight; it varies depending on terrain.
I don't think a bullet would hit point of aim zeroed at 400 yards with all the issues mentioned above.
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I once ran the wind drift calculations from 0 to 1000 by 100 meter bands of wind. I found that the most important winds were from 200 to 600 in terms of total effect in MOA dispersion.
In the first hundred meters, the bullet is going so fast that deflection is minimal, and in the last 100, there isn't enough distance left for the bullet to be deflected much.
I also found that if you had opposite wind conditions at the firing line as between 200 and target, you will miss the target completely if you shoot into the wind at the firing line, as your correction is actually in the wrong direction for the overall wind effect (an intuitive answer that deserved checking).
Anyone who has ever shot "toilet bowl" wind conditions will attest how lucky people are who get uniform wind conditions to shoot in.
Jimro
In the first hundred meters, the bullet is going so fast that deflection is minimal, and in the last 100, there isn't enough distance left for the bullet to be deflected much.
I also found that if you had opposite wind conditions at the firing line as between 200 and target, you will miss the target completely if you shoot into the wind at the firing line, as your correction is actually in the wrong direction for the overall wind effect (an intuitive answer that deserved checking).
Anyone who has ever shot "toilet bowl" wind conditions will attest how lucky people are who get uniform wind conditions to shoot in.
Jimro
I did my calculations using the drift rate (inches per yard of bullet flight) at 300 yards where the crosswind stopped for the last 600 yards of a 900 yard target. The bullet flies straight from 300 to 900 yards at a constant angle to the line of fire.
Then used the bullet velocity at 600 yards to calculate crosswind drift for the last 300 yards to the target at 900. The bullet had no crosswind drift the first 600 yards of flight.
Then used the bullet velocity at 600 yards to calculate crosswind drift for the last 300 yards to the target at 900. The bullet had no crosswind drift the first 600 yards of flight.
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waveslayer
New member
No, same fall. It's the drag that causes the most drop
Stevie-Ray said:
The difference in gravity is very small, probably insignificant. A change air density due to altitude will make a bigger difference.
The cancelation of gravity due to centripetal acceleration of the earth's rotation at the equator only amounts to about 0.1 ft./second squared. The acceleration of gravity is about 32.2 ft./second squared at the earth's surface.
The earth would have to rotate about once every 80 minutes for the centripetal acceleration at the equator to completely cancel gravity.
Unlicensed Dremel
Moderator
As mentioned, there are two primary factors for a given TOF (time of flight): MV and BC (muzzle velocity and ballistic coefficient of drag), and the interplay of the two, cumulatively, with each passing millisecond (since, as a bullet slows, its vel at that moment in time changes, which can slightly affect the BC itself, and since the worse the BC, the faster it slows, and this effect is exacerbated the higher the initial velocity; therefore, the longer a "low BC" bullet is in flight, the less proportional negative effect the poor BC has on it, because it slows down, lessening the effect of air on it).
However, note that this excludes angle of shot (i.e. it assumes a perfectly horizontal barrel). So I suppose you could say that "angle" is one of the components - it is really, but I think you're asking "excluding angle".
And note that BC of a bullet can change with velocity itself - whereas bullet A might have a slightly better BC than bullet B in the "midrange" of velocity (1800-2400 fps), bullet B might have a slightly better BC than bullet A in the "high" (over 2400) or "low" general-velocity ranges.
And yes, ALL of this is how we describe the effect of gravity on an object moving horizontally (initially) in the air.
Also, note that there are some other factors: Air density, which is a function of elevation, bullet spin rate (spin can actually cause the bullet to rise or fall), and possibly others (I don't *think* ambient temp. makes any difference in and of itself; rather, only as a it applies to affecting MV due to increased pressure).
Also note that there are many various factors which in turn determine the two key factors of MV and BC. MV is affected by powder charge, pressure, barrel length, chamber and barrel temp, powder temp (and ambient temp), bullet weight, rifling type, barrel slickness, bullet slickness (coatings, material), bore diameter vis a vis bullet diameter, bullet bearing surface area, etc. BC is affected primarily by bullet shape (which in turn has many components - length to width ratio and others), and to a lesser extent raw weight.
Excluding TOF (which is of course the main factor - giving gravity time to work on the projectile) - in other words, assuming equal time of flight, the factors which affect the fall rate (and total fall, or total drop) the most, in order are:
1. MV (except that at looooong ranges, BC surpasses MV in overall importance; i.e. #1 and #2 here can switch places)
2. BC
3. Rise (or fall) due to bullet spin (think pitcher's curve ball)
4. Air density due to elevation (nitrogen, oxygen, etc. density)
I may have 3 and 4 backwards, but that's relative - a HUGE change in elevation could trump a small change in bullet spin rate; whereas a huge change in bullet spin rate (and thus rise or fall) could trump a small change in elevation.
There may be others I'm forgetting or don't know (Coriolis effect?, etc.) I suppose if you shoot directly AGAINST the spin of the earth, the time before hitting the ground would be shorter than if you shot directly with the spin of the earth, ceteris paribus, no?
Wind is another one, which would normally be excluded from this "drop" conversation / calculation. But direct and strong downdrafts or updrafts, though fairly rare, could obviously affect bullet drop, as could heavy rainfall.
But yes, even the gravity "rate" or level could make a tiny tiny tiny difference. This could depend on whether the moon is directly overhead or directly on the opposite side of the earth (or not), as well as where on the earth you are (gravity is slightly less at the equator, where the earth is "fatter"). But these differences are infinitesimally small, in reality.
However, note that this excludes angle of shot (i.e. it assumes a perfectly horizontal barrel). So I suppose you could say that "angle" is one of the components - it is really, but I think you're asking "excluding angle".
And note that BC of a bullet can change with velocity itself - whereas bullet A might have a slightly better BC than bullet B in the "midrange" of velocity (1800-2400 fps), bullet B might have a slightly better BC than bullet A in the "high" (over 2400) or "low" general-velocity ranges.
And yes, ALL of this is how we describe the effect of gravity on an object moving horizontally (initially) in the air.
Also, note that there are some other factors: Air density, which is a function of elevation, bullet spin rate (spin can actually cause the bullet to rise or fall), and possibly others (I don't *think* ambient temp. makes any difference in and of itself; rather, only as a it applies to affecting MV due to increased pressure).
Also note that there are many various factors which in turn determine the two key factors of MV and BC. MV is affected by powder charge, pressure, barrel length, chamber and barrel temp, powder temp (and ambient temp), bullet weight, rifling type, barrel slickness, bullet slickness (coatings, material), bore diameter vis a vis bullet diameter, bullet bearing surface area, etc. BC is affected primarily by bullet shape (which in turn has many components - length to width ratio and others), and to a lesser extent raw weight.
Excluding TOF (which is of course the main factor - giving gravity time to work on the projectile) - in other words, assuming equal time of flight, the factors which affect the fall rate (and total fall, or total drop) the most, in order are:
1. MV (except that at looooong ranges, BC surpasses MV in overall importance; i.e. #1 and #2 here can switch places)
2. BC
3. Rise (or fall) due to bullet spin (think pitcher's curve ball)
4. Air density due to elevation (nitrogen, oxygen, etc. density)
I may have 3 and 4 backwards, but that's relative - a HUGE change in elevation could trump a small change in bullet spin rate; whereas a huge change in bullet spin rate (and thus rise or fall) could trump a small change in elevation.
There may be others I'm forgetting or don't know (Coriolis effect?, etc.) I suppose if you shoot directly AGAINST the spin of the earth, the time before hitting the ground would be shorter than if you shot directly with the spin of the earth, ceteris paribus, no?
Wind is another one, which would normally be excluded from this "drop" conversation / calculation. But direct and strong downdrafts or updrafts, though fairly rare, could obviously affect bullet drop, as could heavy rainfall.
But yes, even the gravity "rate" or level could make a tiny tiny tiny difference. This could depend on whether the moon is directly overhead or directly on the opposite side of the earth (or not), as well as where on the earth you are (gravity is slightly less at the equator, where the earth is "fatter"). But these differences are infinitesimally small, in reality.
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Stevie-Ray
New member
Interesting. I just found out that the difference imparted on ME at the equator vs, the North Pole is about 1.2 lbs. I would guess, therefore, the difference between the 45th parallel and equator would be somewhere around .6 lbs, as you guys have said, MINISCULE.
This has been quite an interesting thread.
This has been quite an interesting thread.