Travis Barker Doubles Down On Bet With TMZ Photog ... YOU WILL TAT YOUR FACE

Travis Barker ain't backing off the bet he made with a TMZ photog over the big Brock Lesnar vs Mark Hunt fight at UFC 200 ... saying he's even picked out the spot where our poor bastard will have to get his tat.

To jog your memory ... we got Travis out a while ago and asked him who he thought would win the match up ... he said Hunt, our guy said Lesnar ... then the two men bet a new tattoo on it.

We got Barker out in NYC again ... and asked him where our guy would have to permanently mark himself if Hunt wins ... and his answer was pure Rock 'n' Roll.

Check out the clip.