Sir Paul McCartney -- Bodyguard Means Business ... I'll Use My STUN GUN!

Protecting Sir Paul McCartney is no laughing matter -- and last night his bodyguard threatened to open a major can of whoopass on photogs ... in the form of a stun gun.

The altercation went down right after Sir Paul and Dave Grohl were wrapping up a double date for dinner at Crossroads restaurant.

McCartney was stepping into his chauffeur driven ride ... and the bodyguard clearly thought the paparazzi were getting a little too close, 'cause he barked out ... "Don't make me use my stun gun!"