Let the mass of the bullet be $m$ and that of the rifle be $M$. Initially both arc at rest. Hence the total linear momentum of the system $=0$
Now, after the bullet is fired, let the velocity of the bullet be $v$ and the recoil speed of the rifle be $V$, then from law of conservation of linear momentum,
$m v-M V =0$
$\Rightarrow V =\frac{m v}{M}$
The KE of the rifle is
$KE _{r}=\frac{1}{2} M V^{2} =\frac{1}{2} M \frac{m^{2} v^{2}}{M^{2}}$
$=\frac{m}{M} \frac{1}{2} m v^{2}$
$=\frac{m}{M}\left(K E_{b}\right)$
$\because m < M$
$\therefore KE _{r} < KE _{b}$
$\therefore $ Kinetic energy of the rifle is less than that of the bullet.