Of course it isn't an energy multiplier. The energy carried as the bullet leaves the barrel will be very very close to the same. The energy it carries will be lost more quickly to air resistance with a less dense round though. Especially since the same powder behind less mass, means more speed, means more air resistance. How much that matters depends on a lot of factors, but it can certainly be enough to matter.

[–] spankysalmon@fedinsfw.app -2 points 1 week ago* (last edited 1 week ago)

Unless you're a sniper working from 100+ yards, you do not need to care about the energy delivery difference air resistance would make.

Even then, it's more about bullet drop and far less about suddenly losing lethality.