An armor-piercing (AP) round is a type of ammunition designed to penetrate armor, or impact against it and cause spalling of the armor's inner surface, inflicting wounds on those behind it.
From the 1860s to 1950s, a major application of armor-piercing projectiles was to defeat the thick armor carried on many warships. In response to the British Mark I tank, Germany experimented with designs that could penetrate its durable armor. A first design was the "reversed bullet", which struck with a normal bullet's rear blunt face. This was followed by the "K bullet", which used a tool steel core to resist deformation, and used a stronger propellant. From the 1920s onward, armor-piercing weapons were required for anti-tank missions.
Modern armor-piercing shells must withstand the shock of punching through armor plating. Shells designed for this purpose have a greatly strengthened case with a specially hardened and shaped nose, and a much smaller bursting charge. Some rounds also use explosive or incendiary tips to aid in the penetration of thicker armor. High Explosive Incendiary/Armor Piercing Ammunition combines a tungsten carbide penetrator with an incendiary and explosive tip.
— Battlelog Description
AP Tracer Rounds are a weapon Accessory introduced in the Battlefield Hardline: Criminal Activity expansion. Available to both factions for Assault Rifles, Battle Rifles, and DMRs, AP Tracer Rounds deal a greater amount of damage to armored targets and counteract the Armored Insert gadget, but deals less damage to unarmored targets. The accessory also gives each round a visible tracer path, allowing players to track their firing paths easier, although it also exposes their location to the enemy as a result.
As it is a direct counter to Mechanics running the Armored Insert, the accessory is very useful for small maps and gamemodes, like Team Deathmatch and Bounty Hunter, as Mechanics will generally run the gadget in these environments. Due to the accessory's lower damage against unarmored targets and that the tracer will reveal the player's position, it is less useful on larger maps with longer range shooting, like Backwoods and Dust Bowl. Since the round uses the Accessory slot on the weapon, the player will have to choose it over attachments with arguably more utility, like the Laser Sight or Extended Magazine, making more of a situational attachment than one that could be used permanently on a weapon.
When switched to from the Scout primary weapon to the K Bullet, the player character ejects one normal round from the rifle and loads in a single K bullet, effectively replacing the round previously chambered in the rifle. After fired at a target, the user loads another K bullet into the chamber, if available. This means that the K Bullet is functionally single-shot, and has a varying fire rate depending on the speed of the rifle's reload animation (though some rifles have unique reload animations for K Bullets). The Lebel Model 1886 is notable for being the slowest out of all rifles due to needing to pull the bolt twice to use the cartridge lifter. 
If the player switches back to their primary weapon from the K bullet, the K bullet will be simply ejected from the gun (this does not play when the player has ran out of K bullets). The primary weapon will be loaded with one less round compared to before the player switched to the K bullet, and the player's reserve ammo will gain a round, representing the round replaced by the K bullet during the first switch animation. This however does not happen on the M1903 Experimental, which involves removing its attached Pedersen Device and its .30-18 Auto magazine entirely and replacing it with the assembly for the .30-06 Springfield. This also does not happen with the Carcano M91, since switching back to the primary weapon involves the player loading in a new full clip of normal ammunition.
The K Bullet inflicts more damage up close and tapers off greatly. They are capable of disabling light armored vehicles if they hit engines, and deal nominal damage to heavy armor. K Bullets are also effective against elite classes and cavalry, who take reduced damage from normal bullets. While the damage they do is not very significant against most types of vehicles, K bullets halt repairing processes and can help give more effective anti-tank weapons the chance to finish off damaged vehicles.
Any kills earned are credited to the gadget, regardless of the player's chosen weapon. All Scout rifles also inflict similar damage when using K bullets, which lack the one-hit kill zones found on certain rifles. This includes the M1903 Experimental, which has a unique procedure for exchanging the bolt carrier.
- In Custom Gamemodes where primary weapons are banned, a player equipping the K Bullets will pull out a slightly modified Russian 1895 without any attachments to load the K bullets.