[Gatac]

Italy once fielded a machinegun that didn't eject spent cases, but instead fed them neatly back into the magazine. This required an additional automatic greasing of ammo before being fed into the gun.

By all accounts, it worked, even if it was totally impractical.

(Note that similar systems are commonly used for autocannons built into airplanes, where foreign object damage to the engines from flying cases are a very real danger.)

Also, in the first World War, the German Army built the so-called Paris Gun, a massive piece of (barely portable) artillery that could shoot 150 kilometers far. It was an ineffective terror weapon at best, but the French had the gun destroyed and hunted down every technical drawing of the weapon system after the war - presumably afraid of anything of this scale ever being built again.

(Militaria is full of these things. Really.)

Gatac