Dayton James Webber — the quadruple amputee accused of fatally shooting another man in his car last month — apparently got pretty wild with friends when he wasn’t training for his next professional cornhole tournament.

Jaw-dropping video obtained by TMZ shows Webber doing a headstand against a wall and seemingly snorting something off a dollar bill held by another person off-camera … and then eagerly moves over to an open window, lifts a semiautomatic rifle and fires into the night. It’s all in the clip — check it out for yourself.

The wild video was recorded in July last year in La Plata, Maryland … where he’s from.

We’ve told you all about Webber — he’s accused of shooting Bradrick Michael Wells on March 22 during an argument in La Plata while Webber was driving. Webber allegedly dumped his body in a random yard in Charlotte Hall.

Webber was later located in Virginia … where he was arrested and charged with first-degree murder, second-degree murder, and more.

The professional cornhole athlete and gun aficionado ain’t denying he’s got blood on his hands — but his attorneys are arguing he shot Wells twice in the head in an act of self-defense. He was denied bail and remains behind bars.

Webber’s attorneys, Andrew Jezic and David Moyse, tell TMZ … “The video changes nothing. Dayton is 100 percent Not Guilty, and did what he had to do to not be killed.”

We caught up with Webber’s ex-girlfriend after his arrest, who told us she wasn’t the least bit surprised by the serious allegations, even telling us he had an “angry dark side.” She also confirmed she dated Wells after she ended things with Webber, who was aware of their relationship.