LAW rocket launchers only come with 1 shot, after you fire the single shot it is thrown away and cannot be used again (a disposable launcher).
However, if you press Q after it has been thrown away you will switch back to it, it will display 0 ammunition in red but u can still fire rockets and they still have the same effect.
WARNING: If you switch to another weapon in between you can not switch back to the launcher again, for example, if you fire a shot and you automatically select your primary weapon, all you have to do is press Q to switch back to the launcher and fire more shots, but if you switch to any other item in between u can no longer switch back to the launcher.
If you want to select another weapon then select the launcher first and then select the other weapon so u can allways switch back to the launcher with Q. If you have changed your controls then use whatever button u changed Q for