Jeremy Renner has fired back against his ex-wife and damning claims she recently made about the actor's alleged drug use and abusive behavior.

And he's done so in very specific and unexpected fashion.

For those who missed it, Sonni Pacheco is seeking sole and full custody of the former couple's six-year old daughter, Ava.

In court documents obtained by TMZ, Renner's ex-wife cited a multitude of disturbing incidents that she claims make Renner an unfit father who should not be allowed near his child without supervision.

First, Pacheco alleges that Renner was at a club this past November -- high on cocaine -- when he starting talking about killing Sonni, telling someone he "could not deal with her anymore, and he just wanted her gone."

Pacheco also cites to a separate incident in which Renner stuck a gun in his mouth... threatened to murder Sonni... and then fired a bullet into the ceiling... all while Ava was asleep in her bedroom.

Overall, Pacheco says that The Avengers star has a history of substance abuse, verbal and emotional abuse.

She adds that he once left cocaine out in the bathroom, in plain sight of Ava and within easy access of the toddler.

Pretty heinous stuff -- if true.

But Renner says these claims are anything but.

In his official legal retort, Renner now says Pacheco is simply looking to make money off of him.

He points to Sonni as the one with the drug problem and says he has passed every drug test that has come his way over the past three months or so.

Renner then really lays into Pacheco, claiming she'ss obessed with sex, having bragged to friends that she had "bagged an Avenger" and joking she wanted to introduce her pals to fellow Avenger cast members so they could "bag an Avenger as well."

And THEN Renner brings up something even weirder.

The actor -- once again, in his own legal papers -- admits that he and Sonni exchanged naked pictures back in the day.

Over the course of their ongoing custody battle, he alleges Sonni submitted uncensored photos of his penis to their attorneys and the evaluator "for no other purpose than to cause me extreme embarrassment."

He adds that Sonni once threatened to give these same X-rated snapshots to the media.

The documents allege Sonni is simply obsessed with Jeremy's sex life...  and the sex lives of those in his orbit, including one of his nannies. 

The actor claims Sonni has tracked down racy internet photos of his nanny and sent those to the evaluator and his lawyers.

Seriously.

Well, seriously according to Renner, we should say.

In conclusion, Renner explains that Sonni is an artist -- but that her sculptures are wildly sexual in nature. To the detriment of their daughter, who is often exposed to the work.

Jeremy writes that he's deeply concerned Sonni is "exposing Ava to such dark, graphic, sexual material on a daily basis."

Wild, right?

This is what a rep for Renner has told TMZ:

The well-being of his daughter Ava has always been and continues to be the primary focus for Jeremy.

This is a matter for the court to decide. It's important to note the dramatizations made in Sonni's declaration are a one-sided account made with a specific goal in mind.