Shock Lancer

A Shock Lancer (衝槍弾頭(ショックランサー)) is a special type of bullet, that when used, creates a shockwave phenomenon, allowing for an more damaging effect when the bullet hits its target.

Principles
The bullet is designed with a unique grooves upon it, which when fired, uses the air resistance created by the groove to create a "shockwave gun". This shockwave will follow the bullet from behind and strike it.The grooves increases the damage of the bullet by 5 to 10 times, though it will melt due to the air resistance, at the expense of slowing down the bullet.

It is described that the bullet converts the air resistance that is upon the grooves into a shockwave gun, allowing the bullet to move forward like an open parachute.

Three Stories Arc
The bullet is notable for being what cripples Accelerator, making him lose his powers. It is used by Amai Ao several times, but it, and the gun he uses is destroyed after Accelerator manages to retain his consciousness long enough to use his powers to redirect the bullet that was meant for Last Order.

Trivia

 * It apparently only appears in the original light novel, as none of the adaptations mention the bullet used by Amai Ao as being any special.