With my Prodigy, I had some trouble starting the end cap back on after changing batteries. The threads are very fine and both the barrel and the end cap are aluminum so I was concerned about cross-threading the soft metal.
I held the barrel stationary in one hand, Prodigy logo on the side visible. I unscrewed the end cap slowly until it barely popped free. Holding the end cap against the end of the barrel, I used a marking pen to make an index mark on the side of the cap, in line with the tail of the "y" in Prodigy. Then I made a larger, permanent mark on the inside of the end cap.
Now when I replace the end cap, I align the mark on it with the logo and carefully start threading it in. It also helps to push in on the cap with the palm of your hand while turning the Prodigy with the other hand. This compresses the spring and takes some pressure off the threads.