Other games have systems where numerically everything from nanites to the Death Star can take the same amount of damage, it's just way easier to over-damage a fly than a fully grown dragon. Shadowrun used a very dynamic system until at least 3e, where your weapon would have had a code like "9M". You would roll your attack and each success (roll over the threshold determined by how easy it is to hit the target) would raise the M (for medium) to S (severe) and then to D (deadly/dead) and then also the 9 to higher thresholds. The target would have to roll against the accumulated damage of, let's say, 12D with each success over the threshold of twelve reducing the damage over the same steps again until light or maybe not injured at all. What those numbers mean in the game is up to interpretation again, but it handles firearms pretty realistically.
I must admit that I always took stopping power as the amount of energy needed to counter the movement energy of a moving target. To use a non-living example, a huge walker shooting an adamantium bolt at a charging tank that stops it dead in its tracks so that the rear comes up, while the sentry gun on the automated tower would just mint little steel coins on the armour of the tank.
