There is an name variable (AttachSocketName_FPP) in BP_BaseWeapon which is socket name to attach the weapon to. Make sure it corresponds with your socket name for this weapon. Here you can read about sockets and attaching: http://shootertutorial.com/2015/06/08/how-to-add-weapons-basics-equipping/
Another thing you could try is to rotate the rifle mesh in BP_RifleWeapon.