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Language Lessons: TRACE

by David “Norseman” Williams

cover photo from the Forensic Outreach Library

TERM: Trace

ALSO KNOWN AS: ghost trail, vapor trail, gas trail, frost trail, shock wave

CATEGORY: Shooting terminology

RELATES TO:  Exterior ballistics, spotting and shot placement (calling shots).

APPLICATION OF USE: Most handy in an environment when bullet impacts cannot be easily observed, such as long range hunting and target shooting applications.Watching the trace of a round can help a shooter adjust the placement of subsequent rounds.

DEFINITION:  Trace is the visible displacement of air by a projectile as it flies through the air. The phenomenon allows you to visually follow the passage of a bullet in flight. (In extremely simple terms, remember the distortion caused by the passage of bullets in the Matrix movies?

(from the noun)

1. a surviving mark, sign, or evidence of the former existence, influence, or action of some agent or event; vestige: traces of an advanced civilization among the ruins.

2. a barely discernible indication or evidence of some quantity, quality, characteristic, expression, etc.: a trace of anger in his tone.

WHY IT MATTERS: Only hits count, and bullets are expensive. Plus, nothing gets chicks hotter at Nancy’s Squat and Gobble than a ballistician discussing projectile trajectory and associated atmospheric phenomenon.

INTO THE WEEDS: The aka terms listed above are not technically synonymous with each other, but they’re frequently used interchangeably. The effect of a bullet in the air is (very) roughly analogous to the wake of a boat through the water. The disturbance created by the bullet’s passage causes a concurrent distortion of light. It is similar in appearance to the shimmer/haze effect you might see radiating off a hot highway in the distance.

It is difficult for some shooters to pick up trace, at least initially. Any object as small as a bullet, even a big bullet, is moving quickly (and even slow bullets are fast).

Watch the video below – fast forward to about 38 seconds if you want to get straight to it.

A bullet will sometimes cause a vapor trail similar to the white lines you see on the tips of a plane’s wings, but this is actually different from trace.  It is encountered less frequently than trace and usually requires more specific environmental conditions. Essentially there is a low pressure system behind the projectile that is created by the bullets air channel. This low pressure draws in moisture from surrounding air, creating a brief cloud (i.e. condensation) that dissipates almost as quickly as it forms.

In some of the most frigid environments that aforementioned cloud freezes in the air and hangs around slightly longer. This is often referred to as the frost trail, but I’ve fired a rifle in temperatures as low as -30 degrees Fahrenheit and haven’t yet observed it. This could be contributed to a variety of factors. On one hand, at negative 30 degrees, who gives enough of a shit to even look for frosty vapor? On the other hand, range was also a likely a contributing limitation. Not that range affects the science, but flight time does effect the shooter’s or observer’s ability to spot any trace because of the in the limited observation window.

Rifle bullet in flight, Herra Kuulapaa High Speed Ballistics Photography

Depending on how deep into the science you want to get, all of these terms can be used at least colloquially as trace. While they all appear for different reasons and require specific environmental conditions. However, what most shooters are referring to when using this term is actually the disturbance in the density of the air. We already know that air density has a marked, measurable, repeatable effect on the flight of a projectile. Therefore it’s a safe assumption that a projectile has a marked, measurable and repeatable effect on the air it travels through it. Add in just a few, specific contributing factors and this effect will become observable.

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Light is one of, if not the, most important components in observing trace. After all, what you’re actually observing are various refractions as the light bends through the pressure cone as the bullet plows its way downrange. I have found that if the sun is bright and behind the shooter, trace is most easily observed.  This has been attributed by some to the alignment of the light in relation to that of the shooter’s axis of observation; when parallel like this, there’s a more dramatic the bend and a more visible refraction. In theory this would also work if the sun was behind the target, but then it would be shining directly in the observer’s eyes and minimize light sensitivity.

We have some messages out to assorted SMEs to get further perspective on that, and will share when they are available.

The target color or background also allows trace to be seen more clearly. The darker the background, the more observable the trace. Light colored or white backgrounds will also show trace but it takes a more trained eye to catch it. Texture and color patterns in the background will also play a role in detecting trace.  It will be easier to detect on solid color backgrounds with little or no randomness to the patterns; however, a relatively motionless background with repeated patterns (such as a brick wall or window blinds) will make the trace disturbance more pronounced.

Movement is the big killer.  The more motion that’s taking place in the background, the less likely you are to see the trace. Trace is very subtle and fleeting. The more distractions behind it, the less likely it will be detected.

Here are some tips to help you observe trace for the first time or to see it more consistently:

As an observer, position yourself as close to the line of bore as possible without danger of disturbing your shooter. Focus your optics on the target then back off the focus slightly so the target is just a little blurry.  This allows your optic to essentially be focused in a space close to the target where the bullet is moving the slowest and therefore has more drag and a bigger signature. Keep the optic as still as possible and observe the target albeit a little blurry, but concentrate on a single point. When the shooter fires, you will see a shimmer similar to the predator camouflage moving in the trees. This is your trace. It will always appear higher than the strike of the round on target because it’s on the descent portion of its trajectory. In time and with a little practice you’ll be calling shots with a fair amount of accuracy on both elevation and deflection.

I’ve consistently observed my own trace during standard service rifle qualifications, with open sights, no optics, on an issued 5.56 service rifle, as close as three hundred yards. With a little atmospheric luck and the proper application of the fundamentals of marksmanship anyone can learn to do this as well. This results in more accuracy, fewer missed shots and a higher probability of recovering game, eliminating threats, or gathering target shooting data.

Remember, I ain’t had no fancy sciencing schooling.  I just done put a lot of lead, into a lot of sumthin’s, in a lot of somewheres – feel free to weigh in below.

IN SUMMARY:  Trace is any subtle observable indication of a projectile in flight revealing its path to the observer. Trace is repeatable and observable under appropriate conditions. An understanding of this effect with well practices observation techniques will greatly increase the shooter’s ability to correct a first round miss into a second round hit.

QUESTIONS FOR THE CROWD: Have you ever observed trace on the range or when out hunting? If so, did you know what it was or how you could use that little shimmer to your advantage? What commentary would you add for our readers?

Til Valhall! 

Norseman

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About the Author:

Gunny “Norseman” Williams enlisted in the Marine Corps in 1993 and deployed to Mogadishu, Somalia in 1995 as a grunt. He was one of only two SLUGs out of fifteen to make it through Sniper Indoc and attend the USMC Scout Sniper School, graduating on his first attempt and later attending Urban Sniper, Urban R&S, Advanced Sniper, Sniper Employment Officer and the Mountain Sniper/High Angle Shooting Course. As you might expect, after some time ramrodding a Sniper Platoon in the FMF he attended the Formal Schools Instructors Course and was selected as a Scout Sniper Instructor for the First Marine Division. There he was the primary instructor for the survival and .50 SASR portions, specializing in fieldcraft and combat tracking. (Note: it was there at Horno that David Reeder, our editor, met both Williams and Freddy Osuna; they instructed a class together — it was the start of a long and improbable  tale.) Not too much later he received orders to the Mountain Warfare Training Center at Bridgeport. Since retiring he has continued to contribute tot he survival community as a Brother of Bushcraft and is also the HFBIC (Head Fucking Blacksmith In Charge) of the forge at Survival Hardware.