I feel it's worth mentioning that the BAR used full size .30-06 ammunition, and was designed to be fired from hip at full auto (it did have a low RoF, but still) Even when it shifted from, for lack of a better word, 'Assault" rifle into the role of squad LMG it still retained relative controllability when firing on the move, especially when one takes the versions that added pistol grips, larger ammo magazines and compensators to the end of the barrel. that the US didn't adopt/adapt weapons like the Bren or their reverse-engineered MG-42 is a testament to the BAR's functionality, at the very least enough that it wasn't worth it to replace it.
For another, considering the described Tredegar Rifle of GW1 is by all accounts literally a copy of the Enfield SMLE Mk.III, using a Union-sourced gun for inspiration like the M1 Carbine doesn't make much sense, as the CSA's weapon tech and engineering would be sourced from the UK and France, their primary benefactors. Hence, my idea behind the MAS-49.
Here's what I had in mind:
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