Except SL isn't at that advanced level we see by the time Naruto has become hokage, because no such boom and peaceful time has ever happened here. Furthermore, making a "gun" isn't difficult no, overly simplified perhaps but ultimately all you need to do is to stick a piece of metal and gunpowder in a pipe, aim it at something, then ignite the powder.
Building a proper trigger mechanism, empty shell removal, making the actual bullets themselves with the right mixtures inside them, especially if its a rapid fire weapon or sniper rifle of any even half noteworthy accuracy/power...whole different beast, especially in a world where such technology has never been utilized before. Closest things have ever come are simple projectile throwers like the sky shinobi utilized, and one can't really just universally try void that based off one tiny image that probably even Kishimoto himself forgot, no offense.
Besides, he wanted opinions and I gave mine. Obviously he's free to ignore it and do whatever the hell he pleases, too, I'm no nazi.
And if someones still confused over what my opinion actually is, I'll nutshell it; you can't make guns like RL firearms, but you can make a projectile thrower utilizing either manual force, gas or magnetism.