You need to re-read the earlier replies in this thread. We talk about the resetting problem in detail and why the original example chose to make a new sphere for each bullet impact.
For the masking by distance that is really easy. In fact the answer is looking right at you from the images in this thread as well. Look at the top image of my last post. Nice distance based mask from the original location. Just multiply your final ring by that single sphere mask and it will do what you are asking.