Question about the JBM Ballistics trajectory program

[tpcollins]

I use this ballistics chart a lot but I'm confused about the designation for the first bullet drop column. Among various option settings is MOA, Mils, inches, and inches @ 100 yards. If I set the 1st column to inches, it will indicate twice the amount compared to inches @ 100 yards. And if I set it to MOA or inch @ 100 yards, and then set the 2nd bullet drop column to Mil, they match perfectly.

Why does just inches read double and what is it used for? Thanks.

 

[SSA]

I've used those calculators, and just checked. Plugged some inch, moa, mil numbers into 2 of the calculators. They work for me. I didn't see what you describe.

 

[The Big D]

Inches @ 100y is an angle measurement - sometimes called "shooters MOA". It's the angle subsumed by 1" at 100y. So at exactly 100y, it should exactly match inches of drop. At other distances, they won't match. At 200y, the ratio should be 2:1.

It's about a 5% smaller angle than a true MOA. JBM includes shooters MOA because some MOA scopes are actually set up for it. JBM handles it fine as best I can tell playing around with it.

Odd trivia: if pi were exactly 3 (instead of ~5% higher at 3.14...) then shooters MOA and actual MOA would be the same thing.

 

[tpcollins]

I just set the 1st drop column to inches, and the 2nd column to inches/100 yards.

They are different for me - is there a reason why or is my MacBook acting up?

Range Drop Drop

(yd) (in) (in/100y)

100 -0.0 -0.0
150 -3.7 -2.4
200 -11.1 -5.6
250 -23.3 -9.3

 

[SSA]

In the example, the 300 yard numbers will be something like 42" and 14", because 42" at 300 yards = 14" per 100 yards.

 

[The Big D]

Those results are fine. You're not understanding what inches/100y means. It's an angle, not a distance. Read it like this:

At 200 yards, round X will drop 11.1 inches, and I would have to zero 5.6 inches high at 100 yards to be dead on at 200.

Or put another way, if you had a scope that was in shooter's MOA with 1/4" clicks, you'd need to dial between 22 and 23 clicks of drop to hit at 200.

 

[tpcollins]

Those results are fine. You're not understanding what inches/100y means. It's an angle, not a distance. Read it like this:

At 200 yards, round X will drop 11.1 inches, and I would have to zero 5.6 inches high at 100 yards to be dead on at 200.

Or put another way, if you had a scope that was in shooter's MOA with 1/4" clicks, you'd need to dial between 22 and 23 clicks of drop to hit at 200.

Thanks Big D, I almost understand that. I bought a mint used Zeiss Duralyt 2-8 and sent it in to have a target turret installed for elevation, so I could use it for longer distances. The scope has 1 click = 1 cm at 100 meters), and the dial is spaced with designations for every 10 clicks (10, 20, 30, etc).

When I convert the 100 meters to yards, the calculations come up within a few thousands of being exactly in mils. So I should ignore inches and inches/100 yards, and just use the column for Mils as all I want to know is how many clicks up for various yardages? Thanks.

 

[The Big D]

That scope has a click that 1/10th of a mil - in other words, the amount the click moves the impact is 1/10,000th of the distance of bullet flight. It's another angle measurement. Mills also turn out to be unitless (the centimeters moved and meters of flight cancel).

The good news is that you can take a ballistic calculator that has distance to target in any type of units you like - yards, meters, or feet - and as long as the drop is output in mils you can dial in 10x that many clicks (so 27 clicks for 2.7 mils) and you will be dead on at that distance. So you mil scope will work perfectly well in either imperial or metric units. You should never have to convert.

 

[Airborne Falcon]

Very interesting thread ... subscribed, and thanks.

I've been using some of the free Android app ballistic calculators with pretty good success. Things sure have changed in the past twenty years in that regard.