@RichardMcJew:
An intimidating Usopp? That just doesn't seem right. Way too out of character. While we're on the subject, why the need to turn Usopp into a generic gun fighter? Yeah, that'd be cool and original. Maybe Sanji should learn Jeet Kun Do and don a yellow jump suit as well. And Zoro should be an ittoryuu swordsman and wear a kimono and sandals.
QFT big time. The slingshot makes Usopp original, don't take it away, just as much as the Clima Tact makes Nami more original, the Rumble Ball makes Chopper more original, and the Gomu Gomu fruit makes Luffy more original. Would you replace the Clime Tact with a spear? Hell, they're both short ranged and pole shaped, why the hell not? Or why not replace the Rumble Ball with anabolic steroids? Both are drugs that make you stronger with harmful effects when you take too much. Or the Gomu Gomu fruit with a whip? After all, weapons that are stronger are better than just hand-to-hand.
You wouldn't make the Clima Tact a spear because spears have been done, and the clima tact is cooler anyway than any old spear. You don't replace the Rumble Ball with steroids because those actually exist in that exact form and are not nearly as original as all the other fighting styles. And you don't replace the Gomu Gomu fruit with a whip because the fruit is cooler in its relative simplicity, it augments Luffy's personality and general fighting style, and whips have been done to death.
How is a slingshot any different? A slingshot and a gun are completely different - no matter how good a sniper he is, a gun would merely hinder Usopp for quite a while until he got the hang of it as well as he had the slingshot, which he'd been using for years. A gun kind of contradicts Usopp's personality - he's very childish most of the time, and a gun would be making him something he isn't. If he acts like a kid, give him a kid's weapon, just like how Zoro acts like a warrior and uses swords, Sanji acts suave and uses his body like a secret agent, Nami is intelligent and uses a weapon requiring intelligence, Luffy relies on instinct when fighting and had a fruit that allows him to brawl, Robin is a master of assasination and has the best fruit for that, and Chopper uses his body to fight without any real training or discipline because he's still a little kid. Finally, a gun is extremely unoriginal, even a dial-powered one. It's been done by Wiper in ths very series, and other series that had Swiss Army Knife guns are Megaman, Metroid, Devil May Cry, possibly Batman, and Darkwing Duck, and that's only listing the first five things that come to mind.
And my final final point is this: take a good look at Luffy VS Usopp. Tell me that was not amazing and proves Usopp's worth with a slingshot. Now take a good look at Kabuto and the firebird star and say you are not impressed. If Usopp can do something that awesome with a child's toy and can rival snipers that launch exploding baseball-bombs and spit tree-piercing bullets of water, that is far more impressive than if he can do so with a real weapon created to kill. Remember that, as good a writer as Oda is, this is not real. Usopp can not think, "Hmm, guns are stronger, why don't I make one?" because, in the end, it is Oda's choice based on plot, character mindset, and his own creativity what weapon Usopp or any other character will use.
However, I completely agree with Bellamy's statement that a one-time Usopp fight with guns would be cool. Especially if he used them in his own special way, like how Sanji didn't just use the knives to stab as most people would. And then never mentioned them ever again.