An armor-piercing (AP) bullet is a type of ammunition designed to penetrate armor. This is commonly accomplished by building the projectile's core out of hard metals, like tungsten alloys, steel, or depleted uranium, as well as utilizing a softer bullet tip. The softer tip will cushion the bullet slightly on impact with a surface, allowing the core to remain intact and continue traveling to punch through an armored target. While most small arms AP rounds are intended for use against infantry with Body Armor, certain AP rounds are designed to penetrate the armor of heavy vehicles, such as the 7.92x57mm German K-Bullet used in World War I against British Mark IV tanks.
A tracer round is an ammunition type that uses a pyrotechnic charge at its base to make a projectile's flight path highly visible, even in dark conditions. This is useful for better determining the trajectory of a fired round or as a signal to the shooter.
Armor piercing rounds for small arms appear in two games in the Battlefield series. Combined Armor-piercing tracer rounds appear as a weapon attachment in Battlefield Hardline, while K-Bullets appear in Battlefield 1.
— Battlelog Description
AP Tracer Rounds are a weapon Accessory introduced in the Battlefield Hardline: Criminal Activity expansion. Available to both factions for Assault Rifles, Carbines, Battle Rifles, and DMRs, AP Tracer Rounds deal a greater amount of damage to armored targets and counteract the Armored Insert gadget, but gives each round a visible tracer path. This allows players to track their firing paths more easily, 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. It also gives the Assault Rifles and Carbines of the Operator kit the ability to damage heavily armored vehicles like Counter-Attack Trucks and Mobile Command Centers, which can be very helpful on vehicle centric maps like Dust Bowl as the Operator usually has little capability of attack vehicles without using grenades.
However, if there are no enemy players using Armored Insert or there are no heavy vehicles on the map, the only effect of the accessory will be the tracer paths added to each shot. While this can be useful for determining bullet drop at range with DMRs, the path of the tracer will be highly visible when fired with high rate of fire automatic weapons, potentially obscuring the target and more clearly revealing the player's location to enemies. Since the rounds uses the Accessory slot on the weapon, the player will have to choose it over the other utility attachments, like the Laser Sight or Extended Magazine.
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 being fired, 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 ejected 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 en-bloc clip of normal ammunition.
K Bullets 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 wear body armor and 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. Against non-elite infantry, K Bullets are as effective as standard ammunition, and can be a useful substitute if other ammo sources are depleted.
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.