RANGEFINDING WITH MIL BASED SCOPES

Nigel Allen explains how airgunners use the bracketing trick to estimate distances with mil-dot crosshair scopes

Range-finding is essential for airgunners – but there’s a trick if you use a scope with a MIL crosshair. MIL-type reticles are very much in vogue. Many scope manufacturers, like MTC Optics, have their own crosshair designs to incorporate ‘mil’ spacings. (These are not always signified by dots). Airgunners favour mil-dot optics because they tie in so well with a pellet’s trajectory, which follows a more pronounced flightpath than a high-powered rifle bullet. An airgun pellet is also more susceptible to wind drift. With so many aiming points to reference in the sight picture of a mil-dot scope, airgunners can easily make allowances for holdover and holdunder. Also, for windage when they’re lining up the shot.

MIL-DOT AIMING

Yet using mil-dots solely for aiming purposes isn’t really taking full advantage of the concept behind the system. Indeed, it’s likely that most airgunners are missing a trick with such scopes.

The mil-dot system was primarily developed for range-finding in military applications. Here, bracketing a target between the dots provides a rough and ready estimation of its downrange distance. While there’s no denying their effectiveness as a multi-aimpoint crosshair for airgun use, the system also makes it an ideal tool for determining target range.

So, for the airgun shooter, looking down a mil-dot scope offers the best of two worlds: plenty of aiming stadia and an integrated range-finding system!

WHAT IS A MIL?

Mil-dot reticles evolve around a unit of angular measurement known as a milliradian. Or ‘mil’ for short. One mil equates to 36 inches at 1,000 yards. In metric, that’s 10cm (100mm) at 100 metres. Being angular, the measurement value of a mil will change over distance, although its relationship is a linear constant. For example, one mil will equate to 20cm at 200m (double that of the 100m measurement). It’s 5cm at 50m (half that of the 100m measurement), and so on.

For most airgun shooting scenarios, distances are commonly 50 metres or less. However, the mil-dot system still translates well. The table in figure 2 shows the benefit of working in metric values; if you wanted to know the value of a mil at 25 metres, for example, it would simply be 25mm!

Besides dots, mil measurements are often represented by short stadia incorporated into the crosshair’s design. These give you a kind of ruler to measure by. As there are advantages to measuring in smaller increments, advanced reticles like MTC Optics’ SCB2 and AMD also offer the shooter measuring marks in half-mil steps. By halving the measurement reference of a mil, smart crosshairs like MTC’s provide an even more precise aiming and range-finding system.

BRACKETING

If you know the size of your target, you can quickly gauge its distance by simply ‘bracketing’ it between the mil (or half-mil) marks on your scope’s reticle and taking a note of how many of these it spans. Conveniently, you can undertake this as part of the aiming process, making it a speedy affair.

Take this scenario as an example (figure 3). You’re lining up on a bird pest, perched on a branch an unknown distance in front of you. The bird measures approximately 200mm from beak to branch. So, because it ‘brackets’ between a five-mil spacing on your MTC scope’s SCB2 graduated crosshair, you can confidently estimate its range to be 40 metres. How? Through a dead easy formula: target size divided by mil spacings. In other words: 200mm divided by 5 mils… equals 40m.

Take another example – a feeding rabbit. You approximate the top of its body to be 150mm off the grass and can bracket it between three mil spacings. So, 150 ÷ 3 = 50; the rabbit is around 50 metres away. Handily, you can then select the aiming dot or stadia relevant to that distance on your scope’s crosshair to allow for holdover. Easy!

HALF-MIL

The half-mil system comes into its own for much smaller targets, such as the ‘kill area’ of a knock-down silhouette, where bracketing measurements need to be far more precise. As such, half-mil reticles are favoured by outdoor competition shooters, though the range-finding calculation is the same.

Suppose you are aiming at a metal silhouette with a 12mm diameter ‘kill’ disc. If you can bracket the disc between a single half-mil spacing, then the distance of the target from you must therefore be 12 ÷ 0.5 = 24 metres. Likewise, if you can bracket it between three half-mil spacings (i.e. 1.5 mils), then its range must be 12 ÷ 1.5 = 8 metres (figure 4).

Given how easy the milliradian formula is, making a spur of the moment range calculation is quick as well as accurate. However, why not print off the table in figure 5 to keep in the pocket of your shooting jacket for an always-at-hand reminder?

BRACKETING WITH ZOOM SCOPES

As simple as the mil bracketing system is to range-find with, there is something to be aware of if you have a variable power scope. And that is the focal plane in which the scope’s reticle is located. Scopes fall into two types:

• those where the reticle is in the first focal plane (FFP)
• and those where it is in the second focal plane (SFP)

The differences are explained here. In short, with a FFP scope, both the target size and reticle remain in proportion to each other, regardless of the magnification you set. Therefore, you can use the bracketing system to range-find your target on any magnification with these scope types.

However, with a SFP scope, you can only accurately use the bracketing system on one magnification setting: the ‘true’ power. Though this is often 10x magnification, it can vary between scope manufacturers and models, so it’s an important number to know on your own telly. If, for example, your scope’s mil-dot crosshair is ‘true at 10x’, but you always shoot on 12x power, you will have to wind down the magnification to 10x when range-finding using the bracketing method.