Ballistics Tracker

Ballistics Tracker is an attachment that allows you to compensate for bullet drop by aiming your center crosshair at a yellow diamond above the enemy's head. Using this attachment places a smartphone on the underbarrel rail.

Function
The Ballistics Tracker is unlocked at 1,500 kills, being the grindiest attachment in the game, beating out the VCOG 6x Scope. It shows a yellow diamond on top of an enemy's head, and allows you to compensate for bullet drop easier. A side effect of this feature is that it can allow you to spot your enemies easier, and eradicate them before they can see you.

Sniping
The Ballistics Tracker is invaluable for snipers on the edge of the map, because it shows the sniper where to aim. Stay far back, and use the highest magnification scope available, and snipe away. Most weapons also have minimal sound when shot from 1,000+ studs away, so it adds a bonus stealth effect.

Assault
This doesn't just apply to Assault Rifles. You can attach a suppressor with your Ballistics Tracker, and make sure to shoot out all the lights. The enemy may not see you, but you can see them.

Counter
When attaching a Ballistics Tracker, you cannot also attach any canted sight or laser on your sniper, so unless you decide to sacrifice your scope for a CQC sight, your close quarters ability is compromised. This is less of an issue for Assault Rifles, since they can mount an underbarrel laser. The tricky part is though, you cannot mount a laser AND a grip, since that laser will take the other spot rather than the underbarrel spot, so unless you're fine with your recoil, you can't be a hipfire God AND lower your aimed recoil at the same time.

Controversy
The Ballistics Tracker attachment has caused controversy and backlash through the community. Many have likened it to aimbot and hacking. However, it is assured that its only purpose, described above in detail, is to track enemies, and make it easier for long distance sniping. The former can also be done by pressing 'e', so you don't need to perform 1,500 kills to perform that task.

In Conclusion
The Ballistics Tracker is a great sight for aiming for the head, but also has the unintentional side effect of acting as a close quarters tracking sight, similar to the Sniper Spotting Scope in Bad Company 2 or the TDD in Battlefield 4.

Trivia

 * The Diamond was misaligned to the left of the actual target. This has since been fixed.
 * It bears many similarities to the TDD, Sniper Spotting Scope, both from the Battlefield franchise, and the Tracker Sight, Target Finder, and Target Enhancer from the Call of Duty franchise.
 * This was the first attachment to bear no physical appearance, until a January 4, 2016 update that gave it the appearance of a smartphone.
 * The smartphone in question appears to be an iPhone 6, with a custom mounting bracket by StyLis Studios.
 * The attachment appeared to 'float' under the barrel of most guns, with the M4 and M16A4 having the phone block the sight. The only gun that attaches the Ballistics Tracker properly is the Dragunov SVU.  This was later fixed in a ninja patch later in January 4, 2016.
 * Despite claiming to calculate bullet velocity, it does not do the leading calculations on the gun, just the bullet drop.