Why are there firearms on the submarine? Do they open the hatch and shoot at the opponent's submarine in case the torpedos don't work?

The idea that a navy vessel wouldn't be equipped to provide its own security in port or dispatch an armed team on a launch is a non-starter.

Because submarines are still ships that can be boarded. This is more for defense of the crew vs offense.

Firing rounds inside a nuclear submarine? Isn't it like firing a gun inside a plane?

It's definitely a thing you don't do without good reason (enemy boarding action/mutiny) but remember subs are designed to survive nearby explosions underwater; there's redundancy and there's massive amounts of time spent training damage control where your repair or work around damaged systems.

Yes. What's wrong with that?

A 9mm, buck shot, or even a 556 (which they are not using on a sub) is not piercing the pressure hull of a submarine. That's before considering all the other mass of metal and matter within the sub.

5.56 is fine in a sub.

Technically firing a .50 / 12.7mm should be fine for the hull even with API.

Bulkheads are thick.

(Note: SynTech / frangible ammo preferred)

Oh, valid!

Just thinking through Navy issued small arms. Lots of M4s and M17/M18.

I mean, I guess there's more risk of ricochets and equipment damage on a sub than a ship? Regardless, a lot of "don't do that" things become "you might have to do that" things in a combat scenario.

There are a lot of things that don't react well to bullets in a submarine. One Jack Ryan comes to mind...

Primarily in case a KGB agent (incognito as a cook's mate) wants to stop a mutiny lead by a group of officers and aided by the CIA.

Possibility of mutiny, sabotage or just someone becoming dangerous because of personal conflicts or mental illness.

If that's what the mission requires, yes of course.

Of course, other aspects of other missions might also require firearms.

And since they have fucking nukes, all possible scales of violence are literally on the table.

I grew up watching Voyage to the Bottom of the Sea... those guys were always needing to break out the small arms. Apparently lots of rowdy sea critters looking to get in the boat.

Defense while docked at port.

BBC made a fascinating documentary called Perisher on the qualifying process for sub captains in the Royal Navy - it's available on YouTube (in the US at least) https://www.youtube.com/watch?v=I1LF2I3fTbY

Ha ha, poop suit!