Why Is Bullet Weight Not a Factor in Trajectory?

[Mark85304]

I've been playing around with ballistics programs for decades and just realized that the trajectory does not vary with bullet weight for a given caliber. It does, however, vary with changes in starting velocity and BC. I have not noticed this until today and am perplexed as to why bullet weight does not affect a bullet's trajectory. Looking at resulting ballistic trajectories from various programs shows the point of impact at 200 yards with a rifle sighted in at 150 yards is within an inch of each other.

This is a great learning opportunity. I know some of the more seasoned loaders/casters will be able to explain this.

[runfiverun]

take your calculator out to 300 and 400 yds.

[375RUGER]

Looking at resulting ballistic trajectories from various programs shows the point of impact at 200 yards with a rifle sighted in at 150 yards is within an inch of each other.

So it does vary.

[Mark85304]

So it does vary.

The results vary between programs, but not when changing bullet weights within the same program. Within each of programs, a .30 cal 125 grain bullet and a .30 cal 180 grain bullet show the same bullet drop.

[newton]

He's right. Ballistics calculators I tried did the same thing. Not a change from a 100 grain object to a 500 grain object. I guess I have never noticed this before.

However, I think the issue is that all objects drop the same rate. I just tried it with a very small pepper(size of a pea or little smaller, probably weights 3 grains) and a 515 grain boolit I have.

They hit the ground at the same time. I think it has to do with terminal velocity.

[newton]

To add. The B.C. changes the way a boolit reacts to air. If the B.C. does not change, for example testing a feather with a different B.C. than a boolit, the objects fall at the same rate. Its very interesting. Its one of those complex, yet very simple, principals.

I think that what your seeing is that most boolits change their B.C. when the weight changes. But, there are other things associated with B.C. so its possible to have two boolits that weigh different, yet still have the same B.C.

In order for a lighter weight boolit to have the same B.C. a heavier one does it has to be more aerodynamic than the heavier one. That is one thing that a lot of people do not get, a heavier boolit (of the same design of a lighter boolit) will travel farther than the lighter boolit. It retains its velocity.

The smaller boolit has to reduce drag to make up for the inability to retain momentum.

Man! I sound smart! Trust me, I'm not!

[atr]

velocity is the primary factor .....the amount of time the projectile is in the air is the amount of time gravity gets to work on pulling the projectile down...the slower the projectile the more time gravity works hence more drop,,,yes shape of projectile (ballistic coefficient) etc. does have a slight influence mainy due to skin friction as the projectile pushes it way throught the air mass, but its really all about velocity.
high velocity=less time in flight=flat shooting=slight drop
low velocity=more time in flight=rainbow shooting=more drop
atr

[Elkins45]

This is a proof of a classic idea of physics: the mass of a projectile doesn't impact how fast it falls (subject to the effects of air, of course). As a practical matter heavier bullets drop more at longer ranges, but that's only because the extra mass can't be moved as fast given the same cartridge. I can probably make a 200 grain 30 cal drop less than a 130, but I have to fire the 200 from a magnum and the 130 from a 300 Savage or similar. If you could magically take the ability to accelerate them out of the equation you would find that heavy long bullets are actually a better choice for long range due to higher BC and greater retained momentum.

[prs]

The drop of a boolit over distance is chiefly influenced by TIME. For the greatest part, the drop when fired at any given time in the "flight" of the boolit is equal to that amount of drop in the same time frame as if the boolit were simply dropped while stationary. The speed at which an object falls is not directly influenced by its weight. However, we do not shoot in a vacuum or outer space, so given the two boolits are made of same material, the "heavier" one is also often larger in frontal area, thus faces more resistance from the air and drops SLOWER than the lighter boolit given time as the measure. BUT, given distance as the reference measure, the boolit facing more resistance will slow more quickly and thus will take longer to reach that same distance as its smaller sample and thus will fall further in that longer period of time it will take to cover said distance. Clear as mud? Thanks, Professor Newton.

prs

[MBTcustom]

That's why Galileo dropped two different mass balls off the leaning tower of Pisa in 1589. To demonstrate that objects fall at a constant rate.
However, just like it is an oversimplification to think of lead alloy as being composed of only three metals (lead, tin, and antimony) it is an over simplification to think of trajectory as being affected only by mass verses time. Other factors include BC (how easily the boolit slips through the air), humidity, air temperature, wind, and angle of trajectory. Heck extreme marksmen say that the direction of the earths rotation in relation to the direction of the shot even matters (called the Coriolis effect). For most purposes though, the strait simplistic calculation will get you darn close, because theoretically, a mass that is moving at a certain rate of speed will only drop so far in a given amount of time and distance, and its hard for an object to be moved too far from that baseline in less than 200 yards, which is how far most of our shots take place.

[David2011]

One of my favorite shooting subjects! It's all about the time of flight. The weight of the bullet is but a single factor in a complex equation. The mass/weight of the bullet affects its momentum retention but the BC, a measurement of drag, affects the speed retention. The shape of the bullet is actually a huge factor in drop. As speed diminishes, the drop increases. A short, fat bullet like a 150 grain .308 bullet will have a relatively low ballistic coefficient and will shed velocity more quickly than a long, slender bullet like the 142 grain .264 (6.5mm). Here's a chart comparing similar weight Sierra MatchKings, a 150 grain .308 (BC .397) and a 142 grain .264 (BC .580) both launched at 2700 fps. The "drop" figure is the distance a bullet would drop from the time it left the barrel if fired perfectly level from a high platform. Calculations are from handloads.com and corrected for my area at 3700' above sea level. Drift was calculated as a 90 degree crosswind at 10mph and is pretty interesting as well.



This shows why the long distance shooters are going to cartridges like the 6.5mm Creedmoor.

David

[44man]

I cant explain some things, poor at the math but I see things that baffle me with revolvers.
I shoot a 330 gr .44 WLN at near 1350 fps, it drops 35" at 200 yards.
I shoot a 420 gr WFN at the same velocity from my .475 and it drops 18" to 20" or so.
You would think the larger diameter boolit with a bigger meplat would drop more but it is not the case.
Both are sighted at 75 yards at the start.
I can only say it is the weight difference. The BC of the WFN must be awful but it sure gets there so it must retain velocity better. I shot the 420 gr, WFN from the .475 and kept them on a 6" swinger at 400 yards. The holdover is not near as much as the 320 gr 45-70 boolt from my BFR at 1630 fps yet the 45-70 only drops 16" at 200, it does not retain it to 400 yards or 500 meters, (547 yards.)
When I shot long range rifle I never went by figures in books, I actually shot the distances. Nothing ever matched. I do the same with revolvers, I actually shoot and measure.
Calculations and programs are like cell phones to me, never to be understood!

[MBTcustom]

Another factor I didn't mention is barrel harmonics. If your barrel imparts a little upward english on the boolit as it exits, then it will effect the trajectory as well.

[Silvercreek Farmer]

Here's a fun one, drop a boolit and shoot a boolit perfectly horizontally at the same time and according to the laws of physics they will hit the ground at the same time. Simple principal but pretty dificult to execute, Mythbusters came pretty close to it

http://www.google.com/url?sa=t&rct=j...iG1gRH9KP9omWg

[David2011]

I cant explain some things, poor at the math but I see things that baffle me with revolvers.
I shoot a 330 gr .44 WLN at near 1350 fps, it drops 35" at 200 yards.
I shoot a 420 gr WFN at the same velocity from my .475 and it drops 18" to 20" or so.
You would think the larger diameter boolit with a bigger meplat would drop more but it is not the case.
Both are sighted at 75 yards at the start.
I can only say it is the weight difference. The BC of the WFN must be awful but it sure gets there so it must retain velocity better. I shot the 420 gr, WFN from the .475 and kept them on a 6" swinger at 400 yards. The holdover is not near as much as the 320 gr 45-70 boolt from my BFR at 1630 fps yet the 45-70 only drops 16" at 200, it does not retain it to 400 yards or 500 meters, (547 yards.)
When I shot long range rifle I never went by figures in books, I actually shot the distances. Nothing ever matched. I do the same with revolvers, I actually shoot and measure.
Calculations and programs are like cell phones to me, never to be understood!

The time a boolit spends in the barrel is a different influence. When I first shot my Contender this was a big issue. It has a 14" .44 Mag barrel and a 2x scope. All rounds used the RCBS 250 grain KT that actually weighed 255 gr. I sighted it in at 50 yards with some .44 special loads at around 840 fps. The first magnum loads completely missed the target. At 50 yards the magnum loads, which were running right at 1600fps, were almost 2 feet LOW. The heavy gun was not reacting as much to the motion before the boolits left the barrel while the slower Special loads had twice as much time in the barrel, thus twice the time to cause the barrel to rise.

David

[huntincowboy]

Kind of a cool side note here, but if you were to fire a bullet from a perfectly rigid prfectly level rifle and somehow drop a bullet of the same mass the instant the fired bullet leaves the barrel, both bullets would hit the ground at the same time assuming that the ground was also level and the bullet was dropped in the same orientation as the fired bullet i.e. not nose down. Gravity affects all bodies the same assuming similar drag profiles. What makes impacts of bullets different in reality is that we cannot perfectly rigidly hold a rifle or even allow the same recoil effect between different bullet weights. Some shooters come dang close but even a minute elevation change from recoil, say .001" at the barrel would result in a little under a fifth of an inch rise at 100 yds. Very fun to think about!

[44man]

Another factor I didn't mention is barrel harmonics. If your barrel imparts a little upward english on the boolit as it exits, then it will effect the trajectory as well.

I agree it can have an affect, mostly with a rifle. Maybe with my revolvers too, good catch but it is recoil rise in the revolver before boolit exit.
To make it confusing, the 45-70 has far less recoil then my .44 yet drops half as much.
It is a mix of all those things, something not added to calculations that might assume no recoil or harmonics.
Each different gun, gun weight and barrel length in the same caliber will change everything.
I prefer bootstrap calculations---shoot!

[Doc Highwall]

If you take two bullets of the same caliber and same shape like a 30 caliber 150gr. RN and a 180gr.RN, the 180gr. will have a higher B.C. and drop less with the same muzzle velocity.

If you take 2 bullets with the same B.C. and start them out at the same muzzle velocity they will have the same trajectory.

[fishnbob]

[felix]

All true. The first shot I do at the "riva" is at a small rock at the same distance as the genuine target. I move the scope hairs to exactly where the boolit struck. Trajectory is now sighted in for. Repeat for other targets at another location, be that location up or down in elevation and/or sideways from the original target. Deer hunters must mark the scope values for each location where a shot is to be taken. Then any deviation from spec is overridden on the day of the shot simulating any drastic change in distance and/or wind. ... felix