Magnetic Weapons: Almost all of them, in fact. Happiness is a warm coilgun¹. (And a plentiful supply of exchangeable heat sinks, because even fancy superconducting orichalcium coils³ can’t stop the Second Law.)

Handgun versions fire super-tiny flechette projectiles at mind-croggling speeds, because, well, like the picture implies…

…it ain’t the mass, it’s the kinetic energy. And since E = 1/2 mv², they know which variable to concentrate on. Vehicle-mounted and starship versions fire, ah, rather bigger but still uncannily small projectiles at similarly mind-croggling speeds.

These projectiles, incidentally – with the exception of various sluggun esoterica most of which are deliberately slow-fired and intended for purposes not involving doing gross damage – are by and large not fitted with any kind of warhead. Pump enough KE into your k-slug, per Rick Robinson’s First Law, and fitting a warhead to it as well is just gilding the lily.

(Also, not fitting warheads to your most common type of ammunition has the advantage that your magazines stop being a one-hit kill. Photon torpedoes, say, would go up like antimatter-stuffed firecrackers in sympathetic detonation. K-slugs… remain solid lumps of metal. I know which one I want to be sitting on in a firefight, and the US Navy appears to agree with me.)

Footnotes:

1. Well, technically, the modern kind also have vector control handwavium in them to get more bang for your buck, but the idea is the important thing.
2. Not a footnote.
3. Which are also used in the very-high-energy-density batteries that solved the power problem for portable coilguns.