has any one ever taken the lower rail off their vltor vis 3 upper. i have got gunk, build up, and debris all over the external of my barrel and i would like to remove it. if i can just get this lower rail off, i can clean in there. i pull the tabs up but i think there is more to it. i see screws in the back of the rail toward the upper receiver but i don't want to go taking stuff apart before i know what im doing. thanks.