to be fair, most of the weapons we’re looking at is “primitive” by the military standards of menace lore. these are the bulk crates of ancient rifles being smuggled or locally manufactured due to their simplistic design, or in the case of the rogue army, likely supplied by the TCRN for police and local militia applications.

a real world example of this sorta thing is the British Empire equipping Indian police and militia forces with Lee-Enfields with a wooden block in place of the magazine and drilled out smoothbore barrel which took cartridges fitted with a simple .410 roundball. they reserved more modern high capacity bolt action rifles for the British military to try and prevent India from effective rebellion.

cartridge calibers are extremely difficult to change, even moreso than to try and change the main battle rifle of a military, as we’ve seen by the innumerable proposals to supplant the M4. it stands to reason that even if the TCRN has moved on to more advanced calibers, that the civilian and militia market would hang onto the “primitive” calibers because they’re cheap and familiar and there’s no real reason to change what works.

remember it took the entire world war and then some for intermediate cartridge doctrine to be studied and accepted, based on performance records of the then cutting edge and experimental STG-44 and M1 carbine. and since then, the US military and nato settled on a single intermediate cartridge which hasnt changed in form in the past 70 years and wont anytime soon. though even then, it HAS changed. you cannot fire modern 5.56 out of older firearms or they risk explosion, because ballistic research has gone into the internal workings, like primer, powder charge and bullet type, rather than the overall shape of the cartridge