Know Your Basic Ballistics

By The Dutch Open Rifle Team

IPSC rocks! IPSC is dynamic! IPSC is trigger time! Nope, not all the time.

Warning: The following article is about ballistics and probably bone-dry! We're also European, so we work in the metric system. For the Americans, 1 meter = 1.09 yards, and one centimeter = .39 inches.

The goal in IPSC Rifle is not to shoot as quickly as possible, but to hit the A-zone as quickly as possible. Without some basic understanding of the trajectory, the latter is very difficult. This is especially true when the distances vary from 0 to 300 meters, and sometime out to 400 meters!

I often hear IPSC Rifle shooters claiming that, with their optic zeroed at 50 meters, they just have to aim a little lower at targets nearer than 50 meters. Then, they aim just a little higher at targets beyond 50 meters. Unfortunately it doesn't work that simple in the real world.

Once a bullet leaves the muzzle, gravity begins to do its work and pulls the bullet towards the ground. Since the barrel is at a small angle upwards relative to the line of sight of the optic, the bullet travels in an upward direction when leaving the muzzle, below the line of sight. It goes up, crosses the line of sight at the (first) zero distance, climbs up to the maximum distance above the line of sight, and then goes down.

The line of sight is intersected again at the second zero, after which the bullet travels more steeply towards the ground. This can be difficult to visualize, but this theoretical framework is something you need to understand. Please study the diagram below.

What this means is that you have to aim higher at targets close by (to point B), aim lower at targets at intermediate distances (between B and C) and aim (much) higher at targets at greater distances (beyond C).

Variable Distances

There are basically two ways to deal with this during matches with large distances. Which you choose depends on the stages and the different distances to the targets within the stages.

The first way is 'hold over-hold under-hold over,' in which you aim higher or lower on the target, depending on the distance to the target. For an AR-15 in .223 with a first zero at 40 meters (second zero at 220 meters) all hits out to 250 meters are within a margin of seven centimeters below/above the line of sight (i.e. the center of the crosshair in the optic). So, you only have to make minimal corrections in order to shoot solid As.

However, from 250 meters, the drop of the bullet is getting very significant. At 300 meters, the bullet drops to about 25 centimeters below the line of sight. Here you have to aim at the top edge of an IPSC Classic Target to score solid As!

This is even more difficult when the target is not a (relatively large) Classic Target, but only a 60% 'Mini Classic Target' or an 8-inch steel plate! This is where reticles with built-in bullet drop compensators can come in handy. Still, you will have to remember your offset position per target!

The second way is 'click to compensate' for bullet drop at various distances with the elevation turret on top of the optic. You can, of course, adjust for each distance within a stage, but that usually takes a lot of time. In IPSC or 3-Gun, this will be detrimental to your results.

My elevation turret has four standard zeros: 100, 220, 260 and 300 meters. Depending on the stage, I select the zero most suitable for the majority or 'most critical' targets. I correct by aiming lower/higher at the few remaining targets depending on the distances and bullet drop for the selected zero.

For this I use a cheat sheet with ballistic data for all four zeros from 0 to 400 meters. For targets up to 25 meters, I also use a red-dot optic as a secondary aiming device. This gives me maximum speed, precision and flexibility!

Assembling Your Data

To get started with 'click compensation' for distance, there are few things to consider first.

It is important that you know your own weapon/ammunition combination. Factors that influence the bullets path are numerous. The most important ones are, of course, bullet speed and the extent to which the bullet loses speed.

It is wise to measure the velocity of the bullet of your chosen ammunition in your weapon. The factory specifications are almost always (much) too optimistic! The rule of thumb is that a 55gr. bullet with 'hot' factory ammo travels at approximately 940 m/s from an 18" barrel.

In addition, it is important that you know how high your scope is mounted above your barrel. This can be measured with calipers. To do this, I precisely measure the distance between the top of the barrel to the bottom of the scope. Then, I add half of the measured lens diameter and half of the barrel diameter to it.

Armed with this information, you can start playing with one of the many ballistic software programs available. These allow you to accurately determine the trajectory of your bullet. When the ballistic software needs a Ballistic Coefficient, use .25 or look at the manufacturer's exact value.

'What If' Scenarios

The advantage of ballistic software is that you can calculate all kinds of 'what-if' scenarios. For those who do not have the patience for this, I made the pictures below. These are based on a 55gr. FMJ bullet, V0 (muzzle velocity) of 930 m/s and a line of sight/bore axis distance of 66mm (JP 30mm mount on a flattop AR-15).

What you see in this picture is where the bullet (in terms of height!) hits on an IPSC-target at the different distances. This, of course, depends on the zero used.

What can we conclude from these pictures?

1. In matches with distances up to 100 meters, a 100-meter zero is suitable for all distances. Since we have a tendency to aim at the optical center of a target when in a hurry (and we are with targets at short distances!), a zero of 220 meters is also fine! At 100 meters you hit a little too high, but there is more than sufficient margin.

1. In matches with targets up to 300 meters, the 100-meter zero is not suitable as a universal zero. Here, the 220 meters zero is very useful, although you have to correct a lot for bullet drop at 300 meters. On a full classic target, you have to aim at the top edge of the target to be sure of As. With a 60% target, you even have to hold the reticle above the target!

In such a situation I would choose a 300-meter zero, but then I must realize that I'm going to shoot over 60% targets placed between 80 meters and 260 meters if I aim the middle of the A zone. So, contrary to what some rifle shooters are claiming, a 300-meter zero is certainly no universal zero for IPSC Rifle.

Here's a practical example:

I shoot a stage with four targets at 25 meters, six targets between 150 and 180 meters and five plates (20 × 20 cm) at 300 meters. Which zero to choose?

My standard 220-meter zero is useful for the targets at 25 meters. For targets between 150 to 180 meters, I get nice 'high' A hits, but at 300 meters, my bullet has dropped about 25 centimeters. That means that I have to aim about half a plate ABOVE the plate in order to make nice center-plate hits.

How about a 300-meter zero? I can then aim at the plates without any correction, but for nice As on the targets a 150-180 meters I have to aim BELOW the A zone.

I choose the latter, because adjusting the aim "inside" a target is easier and especially faster than aiming above a target in a vacuum.

As it turned out, this stage went smooth and in a great time. I shot 22 As and 3 Cs, and I immediately returned my zero to the universal 220-meter zero.

Conclusion

For IPSC Rifle matches in Netherlands out to 100 meters, using one zero is generally fine. I personally prefer a 40/220-meter zero. Here, I have to aim for the top A-zone at targets close by. At 100 meters, I can aim at the center of the A zone. For intermediate distances, I aim for the shoulder-line of the Classic Target.

I you want to be successful at international competitions with longer distances to the targets, a 220-meter zero is the best 'universal' zero, but it is really worth the effort (and training!) to take into consideration the ballistics involved.

For the best results, you have to start using the adjustability of your scope actively! You will find the BDC turrets like the Zeiss ASV+ a big bonus compared to counting clicks yourself. The ease, and thus speed, of handling and the bomb-proof locking that prevents inadvertently changing your zero during a rough stage are invaluable features.

At the end of the day, being able to compensate for different-distance targets without thinking is the rifle-shooter's goal. The sooner things become second nature, the better. But understanding the basics of ballistics will get you there much faster than simple trial and error.

JP BULLETin

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