Download Thorgrim's BRFedit, open up the weapon .brf (go to the folder of whatever mod you're playing, then go into the Resources folder and look through there until you find the right brf) and then select the rifle, and click the "Edit" tab at the top, and go "Edit > Rotate" then change the 0.0 to 270.0 in the top middle box. (Rotates it 270 degrees along the X axis.)
EDIT: Kamris is on the right track, but you shouldn't need any more screenshots. I've had this exact same problem and I'm certain this fixes it.