Railguns. Also sometimes called coilguns, mass drives or electromagnetic accelerators, though these are different versions of the weapon using the same effect, but in a different way.
The basic idea of the railgun is that of a metal slug accelerated to high velocities using electromagnetic forces. There are several versions of the weapon, all following the same principle.
Two parallel metal rails and a metal projectile. When an electric current is passed through the rails, the projectile completes the circuit and the current flows through the rails forwards, thus creating am electromagnetic field which drags the projectile forward at very high speeds, up to several kilometers per second. This version is the simplest version to build, maintain, and use. Massive amount of heat is generated through friction when then slug is fired, and as such the simplicity of replacing the railgun in case of breakdown, and the simplicity of its firing mechanism, makes it highly searched for.
Unlike the railgun, the coilgun is formed by placing multiple electromagnetic coils along a barer. The coils are switched on and off one after another dragging the slug forward at high speed. This makes it a much more complex device than the railgun, and makes it much more dificult to maintain and use.
The main issue behind building a railgun is building material. Due to the extreme heat generated through friction, a railgun made out of aluminum or iron will fire once or maybe even twice, or perhaps simply brake down before the projectile even leaves the barre. The same goes for the projectile. Instead, the most sought after metal when building both is tungsten, sometimes called wolfram. The heavy, durable metal has a high melting point only exceed by carbon, thus providing a somewhat accessible material. The next main issue is power. In order for the railgun to work at a reasonable efficiency, it requires a lot of energy, however this is a far more accessible problem.
A slug fired by a railgun can reach up to 3000 m/s, mach 10, at which point its kinetic energy, even for a small slug, is comparable to that of a large explosive, with larger slugs easily used as bunker buster. This fact alone reduced the size of the shell, increasing carrying capacity while increasing the firepower and eliminating any risks presented by chemical shells, while simultaneously increasing the range and accuracy by an order of magnitude. When scaled up to even larger scales, the railgun can become a fully fledged weapon of mass destruction. A slug fired at a small fraction of the speed of light can deliver enough kinetic energy equal to a small nuclear bomb, with very large slugs being able to completely eradicate entire cities.