The one where Forgotten Weapons takes a mass driver to the range:
I think I’ve mentioned this “weapon” hereabouts before. It remains interesting-yet-meh, shooting a half-inch diameter slug of steel at seventy some meters per second. This is *really* subsonic (something like Mach 0.2) and unlikely to prove fatal barring a good head shot at close range. Still: it’s early days for this technology. Jack up the muzzle velocity by a factor of ten (require a factor of >100 more energy being dumped into the projectile, a non-trivial task) and they’ll really have something… a “firearm” capable of proper firearm performance, requiring no gunpowder and, by the time it becomes available, largely 3D printable by the average home printer… with the non-printable bits being distinctly non-firearm components, thus unregulated.