A recoilless firearm apparatus for firing at least one bullet of a respective standard cartridge, including a front barrel, a disposable firing activator and a rear discharge opening formed behind the front barrel and aligned with the longitudinal axis of the front barrel. The standard cartridge further includes a casing having an external diameter that is smaller than the rear cartridge-chamber diameter, wherein the casing encloses a sealed inner-casing space that contains gunpowder, and wherein the casing includes a primer. Upon activating the primer, the primer explodes to thereby detonate the gunpowder, forming propellant gasses inside the cartridge that are directed both forward and backward as follows: a) forward: firing of the bullet via the front barrel; and b) backward: pushing, by a recoil force Fp, the casing, being a counterweight to the bullet, to thereby eject the casing from the firearm apparatus via the rear discharge opening.