something that i never understood is this: why the world of avatar never developed fire weapons like in our world? i think that it could add a bit more of challenge to the benders during the crisis of the equalist, I suppose that a machine gun could be something VERY welcome for amon and I don't believe that Hiroshi couldn't be able of design something like that. also, what could wield the non-bender soldiers of the Republic forces to fight? i feel that swords and spear could look a bit outdated in the age of korra.
Aside from that, you're missing the point that Amon purposefully used nonlethal weaponry. Aside from the Pro-Bending Arena--where he just blew up a stage, not any actual people--he didn't even use bombs until the city was already under his control.
Guns would be pretty impractical if used against metal benders who could just deflect the bullets back at the weilder, or bend the gun to make it jam. I suppose they could manufacture platinum guns and bullets, but those would be too heavy to carry.
I guess we can just assume gun powder never existed in the Avatar world. People never really had the motivation to invent guns anyway, since people generally used bending for hunting, wars and stuff like that.
And there are cannons, as well. My guess is that hand-portable smoothbores got brushed off by the silly argument (or by "so what, exactly, is the advantage of this over a crossbow?") and rifles were never invented as a result.
By the way: March of Progress (the fic I was talking about in another thread, in which Asami invents the rifle) has been completed. If anyone likes fanfic, it's well worth checking out.