The Pancor Jackhammer is a bullpup select-fire (semi-automatic and automatic) shotgun manufactured by the Pancor Corporation. While there were some existing prototypes, the weapon was never mass produced. The MK3, or Mark 3, was a third variant protoype of the weapon.
The MK3 is categorized as a shotgun in the Rainbow Six universe. The MK3 only appears in Tom Clancy's Rainbow Six 3: Raven Shield.
Tom Clancy's Rainbow Six 3: Raven Shield
Taking advantage of a bullpup design and 10 round rotary drum, the Jackhammer can lay down a devastating field of fire. However, this weapon becomes somewhat unmanageable during sustained bursts on full automatic.