Although it's unlikely, there is another thing that can cause the cap firing problem you describe.
With the Colts, the hammer will rest on the frame when the barrel and cylinder is removed.
If the hammer is stopping on the frame before it can hit the cap (or at about the same time that it hits the cap) that would cause a mis-fire.
To rule out this possibility, test the gun by cocking the hammer. In this condition, you will probably be able to slide the cylinder back slightly so that there is a gap between the front of the cylinder and the rear of the barrel.
Now, gently lower the hammer down so it stops on the cylinder. In this position, the cylinder should be shoved up snug against the rear of the barrel.
If it still has a gap with the hammer down, the nipples are too short, or the hammer is stopping prematurely on the frame.
If it passes this test, at least that problem can be ruled out. :grin:
zonie