Last I recall there is no command to shoot multiple projectiles at one time. As for cheats, these must be enabled by use of the devmap command, which will in turn activate a notice in the connection screen stating, "Cheats active".
What dark is referring to is something you can do in devmap (with cheats on). If you take the hk69 and bind "give all" to a key you can achive rapid fire hk69 nades by spamming that bind and fire.