Most of the time, a leaking rib can be spotted the instant you drop the barrels in the boiling tank. The air between the ribs expands quickly upon being heated, and a stream of bubbles will issue forth from the point where the leak is. This only lasts for a few seconds as eventually the pressure will equalize and the bubbles stop. Also, you obviously won't see it if the space between the ribs has become completely water-filled so that no air remains (haven't run across that yet). Most of the time you drop a set of barrels into a boiling tank, the cold steel will cool the water and momentarily stop the boiling action and the bubbles can be seen briefly coming from the leaking area. It could be that the leak is just from the front sight. It won't hurt your redone bluing job to drop them back into the boiling tank for a short test to see. You don't even need to get a rolling boil going, just nearly boiling will do the job.
