Avatar: The Last Airbender - The Promise/Fridge

Take a look at those discs that Toph's students bend to knock out the rival Firebenders. We're seeing the very first roots of what would later evolve into Pro-Bending!
Aang runs into trouble because his concept of 4 separate nations cannot accommodate mixed bender families, and he feels that harmony cannot be achieved otherwise. But what have he and Katara been doing all along? And the Gaang as such? Could the realization have given him the idea that the United Republic might actually work? He's already faced the issue at a personal level, now he shows the way in extending it to the rest of the world.

While the scene with fire barrier of the rival Kunyo firebending school's students succeeding only in warming the metal coins Toph's metalbending students shoot at them, making said coins more dangerous to the firebenders than to the metalbenders, is pretty awesome, it does bring up a perfectly good reason why firearms seem conspicuously absent in The Legend of Korra even though the technology level could easily support it (even Sokka had gunpowder in one of the original series episodes). They would be serious Game Breakers against benders. Fire barriers would do nothing to stop metal bullets screaming in at near-sonic or supersonic speeds. Water and earth would fare better, but only up to a point. Scale a gun up, and you get artillery. At some point, any barrier can and will succumb to raw kinetic force, never mind the concussive force of an artillery blast alone, which can kill purely through shock trauma.