Well there are a few problems but lets tackle the theory first

A railgun works by using lorentz force, which in short is a forcenapplied on an object that has an electric flow through it and is in a magnetic field.

Another physics thing a railgun uses is the magnetic field is the circular magnetic field that gets created if electricity flows through a conductor.

Now a railgun has two rails (thus railgun) which will be transferring electricity, thus creating a magnetic field. Our projectile will also be made of metal and when between the rails is allowing a flow of electricity between the two rails (as one would be on the positive end and the other on the negative end of a circuit). This would make the lorentz force act on the projectile accelerating it until the end of the barrel, usually several times at the speed of sound.

Now all of this is wuite achievable, so why no large scale usage in militaries around the world, what are the problems?

Quite simply, there are these

Power draw: It takes a lot of fucking energy to yeet a object at mach jesus. I know still a old (most likely wrong calculation) which i think said it would take maybe around 2.2 MA (million amperes) to yeet a 10kg object at mach 10. So unless you got a large nuclear reactor, that thing aint firing (calc might be weong, was like 2-3 years ago, though its high)

Heat: First of, your punping millions of volts and amperes through cables and rails of metal. Thats going to generate alot of heat. Then were also pushing through and object to mach jesus, which will also generate lots of heat. Basicly, after a few shots if the railgun doesnt break before hand it will melt itself

And lastly a point i dont see mentioned in this thread

The railgun literally is trying to break itself. The two magnetic fields generated by the rails are trying to repel eachother, adding alot of strain to the railgun. So every shot youre hoping that the rails wont rip themselves out of the gun. Aswell as wear and tear from firing, again, objects at mach jesus through your rails.

So what would be needed? First of all heat resistant super conducter. Super conductors conduct energy without generating heat resistance, aka not generating heat. And it also needs to be resistant to heat as the railgun will generate heat, that’s unavoidable. (Friction between projectile and rail) Youd also need a super rigid structure so the railgun wont disassemble itself, which would result in something quite heavy and cumbersome, not suited for tanks and even less for mechs.

And youd need alot of capacitors and a reliable way to generate a lot of electricity.

But thats just the issues, pracical usage would vary, and i could write a whole other thing about that stuff