Well, he might be initially confused. Naruto comes from a world with ninjas, not a modern army base. He could think it's some new kind of enemy stronghold. Maybe he'd try to infiltrate it in his typical ninja way, using stealth and jutsus.
In a fanfic where Naruto finds an army base, he might initially be on high alert. He could use his ninja skills to scout the area stealthily. Maybe he would try to figure out if the army base is a threat to his village. If the people in the base seem hostile, he would likely be ready to defend himself using his jutsus like the Rasengan.
Another possibility is that he discovers a hidden room with a secret about the history of the ninja world. This could be related to a long - lost clan or an old war that has been forgotten. Naruto would then have to decide what to do with this new information, whether to share it with his friends or keep it a secret for the time being.
Some of the soldiers in the base might be scared. Naruto's appearance, with his orange jumpsuit and strange ninja tools, would be really out of the ordinary for them. They might think he's an alien or some sort of super - powered threat.
Well, he might discover a hidden jutsu scroll in the library. It could be a long - lost technique that gives him an edge in battles.
In a 'Naruto finds a gun' fanfic, one possibility is that Naruto, being curious, might initially mistake it for some strange new weapon from another land. He could try to figure out how it works in his own Naruto - like way, perhaps by trying to infuse chakra into it, which might lead to unexpected and comical results, like the gun shooting out chakra - blasts instead of bullets.
The story in the 'naruto finds a tank fanfic' could also take a more comical turn. Naruto could accidentally set off some alarm on the tank and then start panicking, thinking it's about to attack him. Or he might mistake the tank for a really big and strange animal and start trying to communicate with it in his own Naruto - like way, which would be really funny to read.
In the 'Naruto finds an egg fanfic', Naruto may take the egg to his teacher, like Iruka. They might study it together. It could be an egg of a rare creature. Naruto's sense of adventure would lead him to discover more about the egg, and perhaps it will hatch into a cool animal that becomes his new friend or ally.
He could find the sword in an ancient tomb. This tomb might be filled with traps and puzzles that Naruto has to overcome. Once he gets the sword, he discovers that it glows when near an enemy's weak point. So, he starts using this ability to his advantage in battles.
Sasuke would be more aloof about it. He might not show much interest on the surface, but secretly he'd be observing Naruto's every move. If there was something in the base that could help him in his quest for power, he'd definitely be interested.
Maybe Naruto discovers that the weird artifact has some special powers, like the ability to open portals to other dimensions.