It's all in new police footage obtained by TMZ Sports ... which shows a cop pulling over a clearly pissed off Ross in the parking lot behind a Texas Wal-Mart.

The cop explains he stopped Ross because of the excess noise from his car ... and after the officer finds a gun and weed in the vehicle, he questioned the 26-year-old.

The cop wanted to know why Ross had the drugs and the gun ... and Ross was so frustrated by the questioning, he said, "I shouldn't even f**king be in this car!"

The cop agreed ... and when Ross eventually told him he had the stuff in his ride because he and some friends had brought weed to the gun range the day before, the officer scolded him.

"I'm assuming if you play for the Cowboys, you've got a lot to lose," the cop says. "I mean, you've got to think about those things!"

11/7 11:33 AM PT -- 11/7 -- TMZ Sports has obtained the police report in Ross' arrest ... and it turns out, the Cowboys' D-lineman was initially pulled over because he was playing his music too loud.

According to the officer, he "could hear and feel loud music" coming out of Ross' silver Dodge Charger ... and says Ross revved his engine loudly as well.

The cop states in the report that because of the noise and the fact that Ross had temporary plates on his car, he pulled the guy over.

During the stop, the officer says he could smell a strong odor of marijuana from the car ... and, before he searched the vehicle, he says Ross told him there a handgun inside of it.

The cop says during the search of the car, he found a joint, a Springfield XD 9mm handgun as well as a small glass container with marijuana in it ... and arrested Ross on the scene.

According to the report, the marijuana weighed .12 ounces.

2:30 PM PT -- 11/6 -- TMZ Sports has obtained Ross' mug shot ... and the dude looks sad.