A bullet spins as the shot is fired from a gun. For this purpose, two helical slots as shown in the figure are cut in the barrel. Projections $A$ and $B$ on the bullet engage in each of the slots.
Helical slots are such that one turn of helix is completed over a distance of $0.5$ $m$. If velocity of bullet when it exits the barrel is $20$ $m/s$, its spinning speed in $rad/s$ is ________