This tax would actually be dangerous! It would decrease safety because people will be less likely to practice at the range. We know that practice increases safety. People who don't practice may not be able to hit the intruder, or be uncomfortable with their guns. Also, ammunition becomes stale over time. While blowing off old rounds at the range is fine, you don't want to rely on them if you might need them.
It would. The chemicals inside are designed to be reactive, both the primer and the powder, and they will react slowly if left to sit, just like photographic film that you should store in the fridge. Dynamite becomes unstable when it ages, and many modern powders have the same active ingredients. If someone hands me a box of old 45 ACP ammo, I will remove the bullet [sometimes it is welded in place due to corrosion, so it takes quite a bit of force to free], dump the powder, carefully remove the primer [which may be inert, but maybe not], clean the casing, and reload as usual. Look for signs of corrosion on the cartridge.