Why did the EFF used Zeon's 120mm cartridge after the war?

[False Prophet]

I know that the real reason why the Hi-Zack didn't use a 90mm machine gun instead of 120mm was because there hadn't been 0080 to invent it yet. But is there any possible explanation for why the EFF chose to go with Zeon's 120mm cartridge instead of the newer 90mm? I suppose that the EFF captured tons and tons of Zaku 120mm machine guns and ammo after the war, much more than the MMP-80, so it was simply more economical to give the Hi-Zack something that could use the surplus ammo.

[Seto Kaiba]

I know that the real reason why the Hi-Zack didn't use a 90mm machine gun instead of 120mm was because there hadn't been 0080 to invent it yet. But is there any possible explanation for why the EFF chose to go with Zeon's 120mm cartridge instead of the newer 90mm? I suppose that the EFF captured tons and tons of Zaku 120mm machine guns and ammo after the war, much more than the MMP-80, so it was simply more economical to give the Hi-Zack something that could use the surplus ammo.

The real reason is the same reason the Hizack itself exists in the first place... the production company and sponsors didn't care for the Rick Dias and Galbaldy Beta designs, and asked for something else.

From an in-universe perspective... you've got a lot of faith in a franchise that doesn't generally bother to explain the little details when a new model's introduced every two episodes to keep the gunpla line going.

The closest I've been able to find to an actual acknowledgement of the topic, which isn't quite an explanation, is in Entertainment Bible 2: Gryps Conflict. It's mentioned in the MS Development History section of the book that the Earth Federation's finances were a mess after the official end of the One Year War and that Mobile Suit development was effectively halted except for minor improvements to existing models because the Federation simply couldn't afford it. Even after the reconstruction plan was launched and development resumed, this philosophy remained in place initially and development focused on applying the MS research captured from Zeon to Federation designs. The Hizack was the ugly compromise born of those conflicting objectives, and it reused as much technology as it could. The Hizack's use of the 120mm gun was apparently an output of that due to delays in delivery of a new model beam rifle.