Hm, interesting arguments, but not quite enough to convince me. I've only seen the first season of Korra (and all of airbender) so there might be more info on what I have to say (feel free to spoil to motivate response). 

Fire benders could ignite the powder, the point is solid enough and hard to disagree with. Even so, I think this would be highly situational (a bender would for instance not want to set of a gun pointed at it). The precision and heat required would also be extremely high (I'm going by modern cased ammunition), a single flame touching or brushing the weapon wouldn't do much unless we've got ye olde blackpowder muskets. Starting a flame inside a bullet would (I think) require an insane amount of skill, on the level of fire lord or avatar. 

Metal benders would screw with the barrel, once again a valid point, and the result would be devastating for the shooter. But here's something that also applies to the fire bender. Range. 
Looking at the series earth benders seems to require direct contact with their material, the fire bender's flames almost always goes out from their bodies and, not counting the comet-fueled fire bending, seems to die out after about 30 feet (10m). Even water and air seems to have about the same range. 

Anyway, there are a lot of tricks that benders would be able to use against a shooter, but I still believe that their existence is viable. It would also shift the power of our enemies from "benders are the most powerful" to "anyone could F you up"

But yeah, feudal tribe-stuff also sounds fun.