Your PG-9 is a recoilless rifle round. It is fired from a SPG-9 Launcher http://en.wikipedia.org/wiki/SPG-9
The 'booster' on the tail end serves the same function as the ejection motor on the shoulder launched PG-7 family, it projects the round out of the tube of the launcher to a far enough distance where the rocket motor can ignite without harming the gun crew..