1)Move the shells as close as possible to the barrel, which should have already shot the missile.
2)Press and hold E and move the mouse to rotate the shell into desired postion.
3)Freeze the shell, and go the the side of it.Grab the shell with the physgun and crouch-strafe side to side(or if you really REALLY need to, up and down)and try to get atleast 40% of the shell in the barrel.Once you suceed, freeze it in place.
4)Press R with the physgun to unfreeze and it should slide down to the bottom of the barrel.If not, it should be safe enough to move down in there manually.
5)Fire it.