On Q4DM8, if you go behind one of the portals next to the RLs, aim towards the RA, and fire through the portal, there is an angle at which you can create an 'loop' of the RL continuously going through the portal. The rocket will go through about a half a dozen times before it seems to hit some kind of maximum life and explode, like a grenade would.

Playing around on this map there is something wierd about the angles of the shots in those teleporters. It's not as simple as "it comes out at the angle you shoot in", that only seems to be true some of the time. Other times it seems to be off by 90 degrees. If anyone can describe this concretely I would be interested to know how it works.