Understanding Trajectory Card

[rg1911]

It seemed time to create a range/trajectory card since our matches are moving to the 600-yard range, and I've only been able to verify my settings out to 425 yards. I created a card at JBM Ballistics (portion attached); unfortunately I did not find an explanation of what the numbers mean, either on the JBM site or in a 'net search. Let's see if I'm correct:

1. Numbers in the shaded areas indicate bullet drop in inches. I'm assuming (there's that dangerous word) that the value is the total drop at the indicated range, and that it is not a value to be applied to the preceding value. That is, based on my 100-yard zero range, the drop at 350 yards is 24 inches, and not the combination of 24 and the preceding 18 inches, for a total drop of 42 inches.

2. I *think* the numbers in the unshaded blocks indicate total number of elevation clicks for my scope that has 1/4-minute clicks. But with my zero range of 100 yards, there is no drop indicated so the meaning of the 3 eludes me. In addition, my observed 300-yard setting is a total of 20 clicks. The table shows 10, which isn't close. And with an observed setting of a total 25 clicks for 350 yards, the 12 in the block doesn't make any sense.

Can someone please clear this up for me?

Thank you for your help.

Richard

 

[turbo54]

Shaded area is drop, the unshaded is drift.

The indicated numbers are TOTAL drop from your ZERO, *not* drop in addition to the last range.

It appears you've built your card in ".250 MOA" increments, meaning that at 400 yards, you need 30 .25moa units of elevation - meaning 30 clicks on a 1/4 moa scope.

If that is your preferred method, that's fine, but I personally find it MUCH easier to have your unit be 1moa or 1mrad, so the trajectory card reads on in straight moa: instead of 30 clicks, it would read 7.5 moa.

A few other points:

JBM has functionality for muzzle velocity varying with temperature. Additionally, JBM has can calcuate your solution for varying density altitude... Also, speed of sound varies with temperature...

...that is why the solution includes different temperature setpoints.

I "zero out" the velocity vs. temperature variation by specifying a low velocity of X velocity at 0º and X velocity at 100º. This way, JBM's caculation will always be based on X velocity.

 

[turbo54]

In response to your point about the table indicating half as many "clicks" as you have observed:

I suspect you've got a 1/8 moa per click scope.

I really think it would be easier for you to set JBM to indicate in 1moa increments.

 

[rg1911]

Many thanks. I now understand the table. I'm impressed that a full-value 10mph cross-wind will move the bullet that far (14 inches) at 400 yards. Certainly explains some of my misses.

I'll create a new card using 1moa increments. And I really only need it for 5000 feet altitude which is the altitude of the range at which the match is held.

Richard

 

[sled_mack]

Richard,

Read up on Density Altitude. Even though you are at the same altitude, the "apparent" altitude that affects the bullet is different. I've never shot in Colorado, but I would bet that your DA changes a lot more than you might think.

Good luck!

 

[roggom]

Sled is correct, I shoot here and at avg temps (spring and fall) you will be at the 5000 ft in DA, in summer in 110 degrees the DA can be close to 9000 feet, and low as 3000 ft in the winter.

Here is a link to G7, I find it much more intuitive. gseven - Ballistic Program

 

[rg1911]

Sled is correct, I shoot here and at avg temps (spring and fall) you will be at the 5000 ft in DA, in summer in 110 degrees the DA can be close to 9000 feet, and low as 3000 ft in the winter.

Here is a link to G7, I find it much more intuitive. gseven - Ballistic Program

Roggom, many thanks for the reminder of density altitude. I also fly radio control planes from a field at 7,000 feet. However, in the summer, it's not uncommon for the DA to nudge the 12,000 feet mark and the take-off rolls suddenly use a lot more runway.

I'll check the gseven application as soon as I upgrade to the necessary version of Adobe Flash Player. (Since Adobe is dropping or has dropped Flash, I wish web sites would quit using it.)

Richard