Get most out of your existing scope

Definitions

• Mil dot reticle (mil): Mil stands for milliradian, not military. One milliradian is an angle representing a distance of approximately 3.6 inches at 100 yards (10 cm at 100 m)
• Minute of angle (moa): One moa is an angle representing a distance of approximately 1 inch at 100 yards (3 cm at 100 m).
• First and second focal plane:This refers to the location of the reticle inside the scope. This is important to know only if you have a variable magnification  scope. More on that later.

First and second focal plane

The easiest way to determine if the reticle is in the first focal plane is to view the reticle through the scope while changing the magnification setting. If the size of the reticle appears to change as the magnification setting is changed, the reticle is in the first focal plane. If it does not appear to change size, it is in the second focal plane.  It is very likely your scope has a reticle in the second plane. Some high end European scopes have a first plane reticle or give you the choice between first plane or second plane setting.  Second plane reticles requires that you know the subtension of your reticle at different magnification.

Reticles

Reticles come is many different flavors. Some are extremely simple like the Fine Crosshair, others can help you estimate the distance to your target (Mil-Dot and range finding reticles) and help you with your elevation and windage correction. The SVD reticle is a Russian sniper rifle reticle. 

It is important you fully understand your reticle and know the subtension corresponding to the thickness of a post, the distance between the crosshair and the beginning of a thick post, the diameter of a dot on the Mil-dot reticle, the circle on the shotgun reticle.

For example the Nikon Monarch UCC rifle-scope 3-9x40, Nikoplex reticle is a duplex cross hair type of reticle. The subtension of the different elements on the reticle are:

Because 1" at 100 yards is approximately 1 moa these numbers translate directly into moa.

It is a good idea to make a small table like this one to carry with you with all the information for your scope.

For example I know with a 7x magnification I will get a 7 moa (14/2) correction if I put the tip of the lower thick post on my target.

Ballistic table

The next thing you have to work out is the ballistic table for the ammunition you are going to use. I chose the classic 30-06 and lifted the ballistic table from the Winchester website.  The table tells us if we zero the rifle at 100 yards, we will get a 3.2" drop at 200 yards and a 12" drop at 300 yards. A subtension of 12" at 300 yards is equivalent to 4" at 100 yards or 4 moa. With the example above and your scope zeroed at 100 yards and set at 9x magnification, you just need to put the tip of the bottom thick post over the target for a 300 yards shot.

Moving target

To shoot at a moving target you have to figure out how long the bullet will take to reach the target and how much the target will move during that time.

The computations required to determine the hold-off for hitting moving target is a little bit more complicated. Let's take a simple example.  You want to hit a caribou strolling from left to right at 300 yards. Evaluating the moving speed of a target at 300 yards is almost impossible and we have to make an assumption.  We can assume the caribou is moving at 2 mph. You are not going to take a shot on a caribou running at 15 mph. That would be a very challenging and unethical shot.

There are 5280 feet in a mile and 12 inches in a foot. There are 3600 seconds in an hour. Our hypothetical caribou moving at 2 mph will move at approximately 35 inches per second (2 * 5280 * 12 / 3600) or 89 centimeters per second.

Next we have to compute the time the bullet takes to travel to the target. This is complicated because the speed of bullet is not constant. In our example the muzzle velocity is 2925 fps but at 300 yards the speed is 2313 fps. It is a reasonably good approximation to take  the average of the 2 numbers (2619 fps) to get the bullet flight time 300*3/2619 = 0.34364 s.

After squeezing the trigger it takes a small amount of time to release the firing pin and for the firing pin to travel and ignite the primer. On an average rifle it is about 3 ms (or 0.003 s).

Given that our caribou is moving 35 inches per second and it takes the bullet 0.34664 second to reach the caribou, the caribou would have moved about 12 inches or 4 moa between the time you squeeze the trigger and the time the bullet reaches it.

Some readers are going to argue the 30-06 is a slow traveling bullet. So what happens if we use a 270 wsm instead. The muzzle velocity is 3120 fps, the velocity at 300 yds is 2561 fps, and the average velocity at 150 yds is 2831 fps.  Under the same conditions the caribou would have moved a little bit more than 11 inches, or about 3 and 3/4 moa.

It is a relatively safe assumption that hitting a slow moving target requires approximately 4 moa correction especially given the very rough estimate of the speed of the moving target. This is why the distance between the cross hair and the tip of the horizontal thick post on the scope mentioned above is 4 moa at maximum magnification. If the target is moving left to right, just put the tip of the left thick post on your target.  If the target is moving right to left, use the right post instead.

Shooting with a cross wind

Cross wind shooting is very difficult. There are small and cheap devices that can give you the cross wind speed at your current position but the wind down range, 300 yards away, can be different. In a very light breeze of less than 5 mph. the deviation of a 30-06 bullet will be negligible at 100 yards, but will be about 1 moa when shooting at 300 yards.

The table to the right clearly shows the wind deflection increases with the distance with a 15 mph cross wind. This is caused by the bullet losing rotational speed and being less stable in the wind. Also as the bullet speed drops the bullet spends more time to travel the same distance.

Personally I would not shoot at a game at more than 150 yards in a 15 mph cross wind with a 30-06 and maybe 200 yards with a 270 wsm.

Conclusion

I hope this small article will encourage people to look more closely at their scope and reticle. The reticle is much more than a crosshair. It can help you shoot at longer range, correct for wind drift and moving target.

Happy hunting and be safe.

Comments and questions on this article can be posted on this forum.