Machine Gun Kelly -- Sued for SLICING OPEN Bouncer's Finger

These pics are GNARLY -- a bouncer's finger was cut open like a fish after rapper Machine Gun Kelly allegedly threw a bottle at him ... and now, the guy's suing ... big time.

The security guard -- a guy named William Long -- filed the lawsuit in Florida, claiming MGK used a liquor bottle as a weapon to slice his finger during a massive bar brawl back in January.

Long's lawyer James Flynn tells TMZ, the injury occurred after MGK pegged a bottle at Long inside the bar ... and the shrapnel nearly sliced his finger off clean. Just take a look at the pics.

TMZ broke the story ... MGK was arrested for disorderly conduct following the incident -- and boasted about it afterwards, tweeting, "Just spent my entire night in a f**king Florida jail...s**t escaladed way to quick last night, it always does with us though."

According to the lawsuit, MGK's shenanigans put Long in the hospital for 8 days ... a stint that also included 2 separate surgeries.

Long's now suing for unspecified damages -- well in excess of $15,000.