120mm Zaku Machine Gun
MMP-78 120mm Machine gun
The MS-05 Zaku I was equipped with a 105mm machine gun developed specifically as a Mobile Suit’s main armament, however during testing, a larger caliber was demanded, so the MS-06 Zaku II was equipped with a 120mm machine gun. This led to the standard machine gun caliber used by the Principality of Zeon forces becoming 120mm. As a high initial round velocity was one of the main objectives during development, the amount of propellant contained is fairly large compared to the size of the round itself. This meant that with minor changes to the chamber, the working parts of the MS-05 machine gun could be utilized for the 120mm round. Though it is a rather typical machine gun in appearance, many external variants and modifications were commonly seen.
There were several types of rounds supplied for these machine guns that could be used according to operational demands. Though early rounds used a complete one-piece cartridge that contained the warhead, propellant, and casing together, with various types of rounds being supplied as complete units, later demands from the front lines led the the introduction of a two-piece design with separate casings and warheads that could be disassembled and reassembled (though not delivered in this state) to allow some flexibility in round choice.
This also meant that the amount of propellant could also be adjusted, and some Earth-deployed units were able to use this flexibility effectively.
These changes also facilitated rapid conversion of production lines to accommodate the bore up from 105mm to 120mm rather easily.
Source:Another Century's Chronicle: One Year War Vol. 1