Could they, in principle, be effective replacements for bullets and gunpowder?
They’re testing ship-based platforms right now. A few obstacles have to be overcome, like heat dissipation, structure fatigue and power supply.
Yes.You would not to expend a spent cartridge from the breech.You can develop a railgun rifle.
Stationary Gun Platform yes. Power requirement are huge doubt batteries/mobile generator will ever be able to keep up. Not at least in my lifetime.
It all boils down to the size of the power source, and the reduction of resistance. Resistance equals heat, which requires size and complexity and weight to dissipate. Power sources for weapons are usually rather bulky.
But if you could create a zero-point energy source running a small, room-temperature-frictionless electromagnetic levitation bubble areound your rail gun projectile...
... that would be, like, awesome.