Machine Gun

A machine-gun of mass $M$ stands on a horizontal plane and contains a shot of total mass $m$ $,$ which is fired horizontally at a uniform rate with constant velocity $u$ relative to the gun $.$ If the coefficient of sliding friction between the gun and the plane is $\mu$ $,$ and the shot is all expended in time $T$ $,$ find the velocity of the gun $?$

Details and Assumptions:
$M$ $=$ $1$ $kg$ $,$ $m$ $=$ $9$ $kg$ $,$ $u$ $=$ $100$ $m$ $s^{-1}$ $,$ $\mu$ $=$ $0.5$ $,$ $T$ $=$ $2$ $sec$ $,$ $g$ $=$ $10$ $m$ $s^{-2}$