Jackhammer is a weapon appear only in Max Payne 1, it is an automatic shotgun. It replaced by Striker in Max Payne 2.
In game
It is only featured in Max Payne. Along with the M79, grenade and molotov cocktail, it has some of the highest damage output in the game. It is first found in Backstabbing Bastard, off B.B. and his henchmen.
Not many enemies are found wielding the Jackhammer and ammuntion is more scarce than lesser weapons in-game. Only around 150 ammo for the Jackhammer is found throughout the entirety of the game.
The Jackhammer can kill most enemies in 2-3 hits at mid-to-close range and has the fire rate of a Desert Eagle.
In Max Payne 2: The Fall of Max Payne, the Jackhammer cannot be found but is instead replaced by the Striker, another type of automatic shotgun that is very similar to the Jackhammer in terms of statistics.
In Max Payne 3 a slightly altered version of the same weapon is known as the Hammerhead Auto Shotgun and is available in Multiplayer as part of the Disorderly Conduct DLC pack.