CHARLIE SHEEN has been accused of threatening to blow a guy's head off with a semi-automatic machine gun.

The unidentified man was a friend of Charlie's, but they had a bad falling out.  And Charlie supposedly texted a mutual friend saying, "I'll blow his head off with my Super 90."  When the guy found out, he filed a police report.

But another friend of Charlie's isn't buying this.  He says, "The accuser is a dishonorably discharged military person with multiple probation violations who is trying to shake Charlie down for money."