Brian Austin Green Shares Threatening DM From MGK in Megan Fox Pregnancy Beef

Brian Austin Green is exposing a threatening direct message he got from Machine Gun Kelly ... and it looks like the two men are still at odds over a pregnant Megan Fox.

BAG just shared this DM from MGK where the father of Megan's upcoming child tells her ex-husband, "Stop asking when our child is gonna be born. you the FEDS."

MGK goes on to tell Brian to focus less on the pregnancy and more on "that apology you owe me for speaking on my name in public," even bringing TMZ into it ... and he warns Brian, "you chose the wrong one to f*** with."

The rapper then takes some more shots for good measure, telling him to go back to filming cereal commercials and trying to diss him as "mr child actor."

Brian's laughing it off ... and comparing himself to Leonardo DiCaprio.

BAG says ... "I didn't know 'child actor' was something bad. Leo, careful. He may be coming for you next."

The DM is more of the back-and-forth between Brian and MGK ... like when Brian told MGK he needed to "grow up" after Megan discovered upsetting material on MGK's cell phone, resulting in their split.