What is meant by flat trajectory?


July 6, 2010, 05:54 PM
As in "that rifle has a pretty flat trajectory".

In another post I asked about bullet rise, and drop. Since it was explained to me in that post that any bullet starts to drop as soon as it leave the barrel it makes me wonder what is ment by the term "flat trajectory".

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July 6, 2010, 06:03 PM
What it means that compared to some rifles the trajectory is flatter. For example a rifle in 45/70 shooting at say 300 yards may have a rainbow looking trajectory compared to a 308 at the same distance. Why? The 308 is travelling faster and gets to the target sooner, therefore the bullet does not drop as much compared to the somewhat slow 45/70

July 6, 2010, 06:04 PM
Velocity. The higher the velocity, the further the bullet travels while it drops the same speed as any other bullet.


edit: It's amazing that as Tony was posting, I was also going to compare the .45-70 to the .308.

July 6, 2010, 06:07 PM
CoRoMo, that's because great minds think alike:)

July 6, 2010, 06:17 PM
All bullets drop at the same rate. The .308 travels faster so it will hit the ground much farther away than say a .38 SPL.

If two guns are fired parallel to the ground each bullet will hit the ground at the same TIME regardless of caliber. The bullet traveling faster will have traveled a greater distance.

So back to the question, yes a .308 can appear to have a flatter trajectory for a given distance because it arrives at the target sooner than the .38 SPL. The .38 SPL will have been airborne for a longer period at 100 yards than the .308 bullet.

Simple physics.

July 6, 2010, 06:22 PM
The bullet does begin to be influenced by gravity the moment it leaves the muzzle. However, remember that the bullet isn't being fired perfectly horizontal. In fact, it's being fired at a slight upward angle.

The comparison between the .45-70 and any military cartridge of the 1900's is about the purest comparison. The .45-70 uses a heavy bullet fired at relatively slow speed. It is capable of very long range shooting and the projectile usually still has sufficient energy left when it reaches the target; however since the bullet spends more time being influenced by gravity, it has the long range trajectory of a rainbow.

At the time it would have been in service, the .45-70 would have used a 500 or 405 grain bullet at around 1,500 to 1,800 fps. Compare that to something like a 150 to 180 grain bullet at 2,500 to 2,800 fps, which would roughly give you an idea of the military rifle cartridges of the world during WWII.

To make a rough generalization which may result in criticism; the more velocity, the flatter the trajectory. There are, of course, other factors which I will leave alone.

July 6, 2010, 06:22 PM
So something like a 5.56x45 round with its high velocity will have what is considered a flat trajectory over something like a 7.62X39. Correct?

July 6, 2010, 06:24 PM
It is a generalization derived from several items, primarily high velocity (which lets the bullet travel farther during a given amount of time) and favorable ballistic coefficient (which lets the bullet overcome air resistance thus staying faster, longer).

A bullet that doesn't move very fast and which slows down quickly (say a mild .45-70 load) must be fired at a relatively sharp initial upward angle to account for how much time the bullet needs to get where it's supposed to go. If a bullet will take (for the sake of discussion) 2 seconds to cross a given distance, it is going to fall downward something like 96 feet before it hits. *** So you'll have to aim the gun upward enough to launch that bullet 48 feet above the target at roughly the midpoint (actually a bit past the midpoint) of its trajectory. If it left a trail in the air you could see, it would look a little like a rainbow.

(*** Anything dropped or thrown falls towards the earth at a rate of acceleration about equal to 32 feet per second, per second. Thus, 32' in the first second, 64 additional feet during the second second, 96 feet in the third second and so on.)

If, on the other hand you fire a bullet a lot faster and it doesn't slow down as fast because it has a good "BC," (lets say this is a .220 Swift, or a .260 Remington or a .338 LM or other pretty zippy round), maybe that bullet will cross the same distance in only one second. If that's so, you'll only have to point your rifle barrel high enough that the bullet will be about 16' above the target at the midpoint of its path. It is still going to fly in a rainbow or arc parth, but a much flatter one.

Hence a "flatter" trajectory.

Said another way, if you watch a quarterback throw a football to a receiver, he throws it way up in the air and the receiver catches it as it comes back to earth. If you watch a 3rd baseman throw a baseball to 1st base, it will almost appear to follow a straight line. A person can't throw a football as fast as a baseball, and it's bigger -- creating more drag and slowing down faster. To get it to fly the same horizontal distance you have to aim it pretty far up in the air. The baseball has a "flatter" trajectory.

Make sense?

(Yes, the numbers are mighty rough. Shouldn't need a ballistic chart to explain the principle, I don't think.)

July 6, 2010, 06:25 PM
So something like a 5.56x45 round with its high velocity will have what is considered a flat trajectory over something like a 7.62X39. Correct? Absolutely, yes!

July 6, 2010, 06:31 PM
Makes perfect sense. Explained so that even an idiot like me can understand, thanks.

Also, its relative isn't it? As in "that round has a flatter trajectory than this round". I think I've got it.

Again, thanks for explaining the bullet rise question that I posted a while back.

Thanks for all of the replies.

July 6, 2010, 07:07 PM
Apparently, the .45 GAP has a flat trajectory out to 300 yards as I've been told by one "expert" that he used no hold over to hit balloons at that range. :rolleyes:

July 6, 2010, 07:13 PM
Graphical comparison of the 45-70 and 308 trajectories.


Let's not forget the BC component, not only velocity...a more aerodynamic bullet will shoot flatter shot at the same velocity.

In the comparison with the 308, a 45-70 shoot a slower bullet with a much worse BC.

July 6, 2010, 07:34 PM
Absolutely, yes!
I think he's got it!

pete f
July 6, 2010, 07:39 PM
Its like throwing a football vs a baseball, you can get a nice flat throw with the baseball because its smaller and lighter, the football needs air under it, (unless your Brett Favre,) to go as far. ITs a combination of size, weight, speed,

The Lone Haranguer
July 6, 2010, 09:24 PM
To use a baseball analogy, the line drive has a flatter trajectory than a pop fly.

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