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The M - 1 Garand was opted due to the sheer number available post world war II and its ability to punch through the jungle and touch someone. I'll have a look at the M - 1 Garand carbine and am also open to suggestions.
IOTL, the US supplied a little under a quarter million Garands & a little under 800k M-1 Carbines to South Vietnam, as well as a small number of M1917 Enfields. Also, the M-1 Carbine is an entirely different weapon that uses a different mechanism & only shares a couple screws in common with the Garand rifle.
Not sure what else could be available, as beyond the mix of French & Japanese gear they would have inherited, there's not a whole lot of stuff being produced for export right after WW2- the British & Commonwealth could probably scrape up some surplus Lee-Enfields, there's probably some ex-German stuff available, & Mauser 98-type rifles from FN in Belgium & Brno in Czechoslovakia (at least until the Soviets force them to cease production for export & start building Soviet types for Warpac use), & possibly some AG-42s & Mausers from Sweden.
Until the likes of the AK-47, SKS, FAL & G3 hit the export market in the late 50s & early 60s, most countries without intact domestic arms industries got their infantry weapons from WW2 surplus supplied as military aid from the US, UK/Commonwealth, or Soviets depending on political alignment, what was left of their pre-WW2 arsenal, & captured Axis equipment.
Actually after further consideration I might consider having a USN or RN officer loaned to the Vietnamese military.
Thoughts?
That'd probably be better for a Western-aligned country in the late 1940s- French loaners would probably be too awkward for Vietnamese domestic politics, & having ex-IJN mercenaries hold that high of a position would cause all sorts of political problems both domestically & abroad. Having them as advisors, staff officers, captains, & shipboard officers might be one thing, but running the whole show quite another. Nor would having ex-IJN officers have a hand in the formation of strategy, doctrine, & institutional culture be a very good idea.