14.5 vs 16" trajectory

[ridgerunner70]

Is there much difference between the 14.5 and 16" barrels on trajectory? I realize that the longer distance would be but, say 0-300yds?

[glockshooter]

Barrels do not decide trajectory. The bullet weight, BC, velocity, zero, and a few other things do.

Matt

[Odie Dozer]

Barrel length slightly affects velocity, however, there isn't going to be a huge difference in the two from 0-300.

[nimdabew]

Don't over think it. Shoot your gun and see what your POA/POI is for different distances.

So say we all!

[Tactical Joke]

Short Answer: Barrel length should really only affect trajectory in that there is a change in velocity. The smaller the velocity change, the smaller the trajectory change.

Geek Answer: Wikepedia has a decent treatment of trajectories. It helps, of course, if you are familiar with differental equations, since drag is variable depending on the speed, and air density, at each point in the path. If I'd realized I could use differential equations to shoot things, I'd have paid a lot more attention in the course.

[Arctic1]

Barrels do not decide trajectory. The bullet weight, BC, velocity, zero, and a few other things do.

Matt

Zero does not affect the ballistic trajectory of a projectile. Zero is an aiming technique, ie marksmanship. It has nothing to do with the physics, it is merely an indication of when a projectile crosses the line of sight. This can happen twice or not at all, depending on zero range.

Example; if you have two identical rifles (M4's for example), shooting the same ammunition (M855), with the same atmospherical conditions (ie same temp, same humidity, same barometric pressure), the trajectories will be identical even though they are zeroed at different ranges.

Let's say that one rifle is zeroed at 200 meters, and the other is zeroed at 300 meters. While there will be tangible differences in POA/POI, the trajectories for both rifles to hit the same spot at 250 meters will be identical.

There are only two constant forces acting on a projectile, gravity and drag (air resistance). The former is constant, and the latter is not relevant due to the ammo being the same, with identical BC and velocity.

Atmospheric conditions can affect the trajectory, if the barometric pressure, humidity or temperature changes, so will the trajectory.

In addition, wind and spin drift will affect POI laterally.

----------------------------------------------------------------------------------

To answer the OP, I did a calculation with the following velocity parameters:

http://i44.photobucket.com/albums/f41/Mazer18/velocity.jpg

BC of 0.304 (M855 BC) and a 300 yard zero with 62 grain bullet.

Apex of trajectory in relation to LOS for a 16" barrel is 6.2" from 144-173 yds
Apex of trajectory in relation to LOS for a 14.5" barrel is 6.6" from 147-171 yds

I set the height over bore value for the sight at 0"

Basically a 1 cm change in POI in relation to LOS.

I used the ballistic program found here:

http://norma.cc/sv/Ammunitionsskolan/Ballistik/

Choose "Balistik US" on the left.

[glockshooter]

I agree your zero does not effect ballistics but your zero does absolutely effect your trajectory. If you have two identical guns and ammo but one has a 25 yards zero and the other a 50 yard zero your trajectory will be different. Whether it is tangible or not depends on what youre doing. There is a difference of more than 4" at about 150 yards.


Zero does not affect the ballistic trajectory of a projectile. Zero is an aiming technique, ie marksmanship. It has nothing to do with the physics, it is merely an indication of when a projectile crosses the line of sight. This can happen twice or not at all, depending on zero range.

Example; if you have two identical rifles (M4's for example), shooting the same ammunition (M855), with the same atmospherical conditions (ie same temp, same humidity, same barometric pressure), the trajectories will be identical even though they are zeroed at different ranges.

Let's say that one rifle is zeroed at 200 meters, and the other is zeroed at 300 meters. While there will be tangible differences in POA/POI, the trajectories for both rifles to hit the same spot at 250 meters will be identical.

There are only two constant forces acting on a projectile, gravity and drag (air resistance). The former is constant, and the latter is not relevant due to the ammo being the same, with identical BC and velocity.

Atmospheric conditions can affect the trajectory, if the barometric pressure, humidity or temperature changes, so will the trajectory.

In addition, wind and spin drift will affect POI laterally.

----------------------------------------------------------------------------------

To answer the OP, I did a calculation with the following velocity parameters:

http://i44.photobucket.com/albums/f41/Mazer18/velocity.jpg

BC of 0.304 (M855 BC) and a 300 yard zero with 62 grain bullet.

Apex of trajectory in relation to LOS for a 16" barrel is 6.2" from 144-173 yds
Apex of trajectory in relation to LOS for a 14.5" barrel is 6.6" from 147-171 yds

I set the height over bore value for the sight at 0"

Basically a 1 cm change in POI in relation to LOS.

I used the ballistic program found here:

http://norma.cc/sv/Ammunitionsskolan/Ballistik/

Choose "Balistik US" on the left.

[Arctic1]

@glockshooter:
If you have a rifle zero'd for 200 yards, and a rifle zero'd for 300 yards, is the angle of the bore different or identical when shooting to hit the same point on a target at 250 yards with either zero?

[Clint]

Wow, a lot of obtuse answers here...

The trajectories from those barrels will be very close, with the 16" being very very slightly flatter.

Run a ballistic calculator at the two different velocities to see exactly how close.

[Arctic1]

There is about a 1 cm shift in POI, with the 16" being a bit flatter, as you say. See my post above for an example.

[glockshooter]

They are different. They are not substantially different at 250. At 250 the difference is about .6". A 300 yard zero is nearly 5" high at 200. It also drops like a rock past 300.

Matt


@glockshooter:
If you have a rifle zero'd for 200 yards, and a rifle zero'd for 300 yards, is the angle of the bore different or identical when shooting to hit the same point on a target at 250 yards with either zero?

[Arctic1]

They are different. They are not substantially different at 250. At 250 the difference is about .6". A 300 yard zero is nearly 5" high at 200. It also drops like a rock past 300.

Matt

They are actually identical.

When most people talk about trajectories, they are actually referring to a given parabolic projectile path, with a specific near zero and far zero. This parabolic projectile path is decided by the following factors:

-Horisontal sighting plane - Height over bore
-Zero range
-Velocity
-Projectile weight
-Projectile BC

For most people, the last three factors cannot be changed.

However, this projectile path does not really say anything about the actual trajectory of the bullet, ie the bore axis angle, only that your POA/POI are the same at two different ranges.

That is why parabolic trajectories created by a ballistics program will look different, even with identical guns, when you change the two first factors mentioned above (or any of them actually, but we are talking identical factors for simplicity).

When you zero a weapon, you agree that you do not change the physical characteristics of the weapon, right? You only adjust the sights. To shoot further out, you adjust the sight by pointing the line-of-sight downwards. And therein lies the percieved trajectory difference: the difference in angle between the line of sight-axis and bore axis.

In order to maintain a horisontal sighting plane, we adjust the angle of the bore axis. This allows us to compensate for gravity. However, gravity does not differentiate based on zero ranges.

A given projectile, weighing a certain amount, traveling at a certain velocity and with a certain ability to resist drag, needs a certain bore axis angle to reach a given distance, regardless of zero.

If you have a weapon with a 200 yard zero, and a weapon with a 400 yard zero, and you are aiming for the bull's eye at a target located at 300 yards, you need to adjust your hold. You cannot change gravity, you cannot change the bullet's properties. When adjusting your hold, you are in fact just increasing or decreasing the angle of the bore axis.

With identical weapons, identical projectiles and identical weather conditions the bullet trajectory to reach a certain distance is also identical, regardless of zero (POA/POI calculation).

The same goes for canting. Canting is shooter error, not a physical problem with the rifle. We all know that when canting, you will hit low to the side you are canting, ie cant left, hit low left.

This happens because of gravity and barrel rotation.

Due to the rotation of the bore axis, while still using the same line-of-sight, the projectile will not reach the same vertical height as an uncanted bore, but the projectile is still affected the same by gravity. Therefore the bullet will strike below the line of sight.
The cant error is due to the angle difference between the bore axis and line-of-sight axis now acting as windage compensation, instead of elevation compensation. Therefore, you hit laterally of your POA.

Let's say that you have a rifle with a 200 yard zero. You have trued it. Now, if you cant this rifle 45 degrees to the left, using the same line-of-sight, you will hit low and to the left. However, if you took the same weapon, removed the sights and canted it, whilst maintaining the same bore axis angle, you would still hit the center of the target.

This perception of different trajectories has everything to do with POA/POI configurations with a certain weapon/ammo/optic combination in regards to the line-of sight. Bore axis angle is never mentioned.

[Nightvisionary]

Since it has to be 16 inches it might as well have 16 inches worth of rifling;)

[Arctic1]

Since it has to be 16 inches it might as well have 16 inches worth of rifling;)

Didn't quite get that comment....

[Packman73]

Didn't quite get that comment....

I think he was saying that the overall length has to be 16" so just get a 16" barrel for the extra rifling over a 14.5"?

[Todd00000]

Great info.