Reading Ballistic Data

[cariboo_kid]

I need help understanding ballistics tables for commercial ammo. I recall a thread where it was explained months ago, but am unable to find it. If someone has a link to it or another page on the net that will explain it to me, that would be great. Otherwise maybe you could give me a brief explanation here.

My confusion comes from the fact that the difference in bullet height (or drop) is different for a zero at different ranges. Just pulling numbers out of the air, if a load sighted at 100 yards drops 2.0" at 200 yards according to the table, the same load from a 200 yard zero is not listed as 2.0" high at 100 yards. The difference will be even more noticable at 300 or 400 yards. This must relate to MOA rather than measuring in inches, or something along those lines.

I completely understand that these tables are not gospel, and every rifle will have different results. I'd still like to be able to read the data and have a starting point that will get me on the paper with fewer wasted shots when trying a new load at longer distances.

Here is the data of a randomly selected federal load as an example, showing distances of 50, 100, 200 and 300 yards:

100 yard zero: -0.3 0.0 -3.2 -12.9

200 yard zero: 0.5 1.6 0.0 -8.2

With a 100 yard zero you'd hit 300 yards with a vertical difference of -12.9 from the 100 yard impact.

With a 200 yard zero you'd hit 300 yards with a vertical difference of - 9.8 from the 100 yard impact.

Why?

 

[Suputin]

The ballistic arc is not symmetrical because the bullet drops more quickly the longer it is in flight.

You don't need to know exactly why the numbers in a ballistic table vary the way they do. The important thing to understand is what the data represents and how to use that data to hit the target.

If you are interested in the math involved, it is quite complex to say the least.

 

[cariboo_kid]

I guess it's due to the slightly different angle that the rifle is held at, changing the arc and giving gravity and wind resistance a different amount of time to affect the trajectory?

 

[421]

When you aim at a target there is an angle between the horizontal and your barrel. As your zero range increases that angle increases and your trajectory changes.

When you fire the bullet follows a path which roughly defines a parabola. If you use the same bullet and load at the same zero each bullet follows much the same path. But when you change your zero, especially lengthening your zero range you increase the angle between our barrel and the horizontal, this changes the parabola.

At some point you will be beyond the high point of the parabola and be on the downside of the arc. This is the reason you can zero at about 200 yards with a .308 and have the rifle still zeroed at about 25 yards. The bullet path is much the same on either side of the parabola, except its going up at 25 yards and down at 200 yards.

Take a piece of paper and graph the trajectories in the bullet data. Think about how you must increase the angle of the barrel as your range increases. As the angle increases the parabola gives you a longer overall range until you reach 45 degs ( which no one shots at exept artillery and motars). You'll find the trajectories start to make sense if you look at the path the bullet follows in that manner rather than just a set of numbers. The first time I looked at the numbers I was a bit confused until I started to think in terms of angles and paths, then it made some sense.

Good luck.

 

[Juster]

>the fact that the difference in bullet height (or drop) is different for a zero at different ranges.

Not a fact. The drop from the axis of the barrel is the same, regardless of the sight settings.
Changing the axis of the sights changes the drop from the axis of your sights (line of sight).
http://www.ballisticsimulator.com

 

[Suputin]

I guess it's due to the slightly different angle that the rifle is held at, changing the arc and giving gravity and wind resistance a different amount of time to affect the trajectory?

Think of how a bullet would behave if you pushed it off the side of a table. It starts out not moving at all but then gravity takes over and it begins to accelerate downwards. It constantly accelerates, gaining speed with time till it hits the ground

A bullet in flight is affected by gravity in exactly the same way as a bullet pushed off the side of a table.

Thus the ballistic arc is not symmetrical because at first the bullet falls towards the ground slowly but over time it falls faster and faster, causing the ballistic arc to be more curved as it moves away from the gun.

 

[beretta boy]

The explanations as to "why" are all pretty much valid.

To achieve your goal of "wasting fewer shots", get into Remington's web site under ballistics, and download their " Remington Shoot" ballistics calculator.
It, of course features Remington's line of loaded cartridges, but if not identical to yours, you should be able to find something very comparable. One of the nice features is that you can change the velocity values for a given commercial load to more closely match your own loads, then re-input the data. You can change the target distance and/or "0" distance at will - say to find where a bullet will be when sighted 2.5" high at 100 for 50, 150, 250, 300 etc., and in one of their drop down boxes, print of a "custom" ballistics table for your load, based on your sighting criteria.

It takes about 10-20 minutes of fooling about to achieve, play around with and learn, but this free software is a definite help. I keep it handy as in icon on my desk-top to get to whenever I need it.