UFC Fighter Ronda Rousey -- I Destroyed My Knuckles on 'Expendables 3' Set

UFC badasss Ronda Rousey learned the hard way Hollywood can be a dangerous place -- and she's got the bloody knuckles to prove it.

Rousey -- the reigning UFC women's bantamweight champion -- just got back from filming "Expendables 3" with Sylvester Stallone in Bulgaria ... and tells TMZ she got some battle scars in the process.

Rousey says while she was shooting the movie -- in which she plays a gun-toting killing machine -- the calluses on her knuckles went away, which made her hands softer and more vulnerable to ripping open.

FYI -- generally, fighters enjoy having calluses on their knuckles, because it toughens up their hands for contact.