Because we are short on designs here's a repeat.
A new semi automatic rife in .25 Remington Auto-Loading
Weight: 4 KG
Length: 1,000 mm
Barrel length: 500 mm
Action: Gas-operated short-stroke piston
Feed system: 20-rounds in a detachable double-column box magazine
Cartridge: .25 Remington Auto-Loading
Rate of fire: 450 to 550 round/min
Features: Integral folding spike bayonet
I would suggest we don't go into new ammo types when we already have a suitable caliber: the
7.62x38mmR used on the Nagant M1895 revolver. Yes, it is a pistol round, but it's actually longer and more powerful than the .30 carbine (7.62x33mm) - closer to the .32-20 Winchester and .32 H&R Magnum.
If you consider rechambering it, I'll vote for it.
Additionally, we could try and assist our Brazilian comrades in overthrowing their warmongering fascist government, or at the very least in creating a chaos: I'd suggest developing a suppressor for the revolver itself, and smuggling as many of them as we can to the Brazilian Communist Party - along with someone who can give them pointers on political assassination and urban guerrilla tactics.
Meanwhile, our own intelligence services might have a use for it.
Just that. It could be made bulkier, but must kept as short as possible to reduce chances of detection
Finally, I'm not certain that we can use a 105mm monster on the POAC frame - it's just an armored car, on the smallish side, and it must still float, you know... Anyway, comparing the POAC-B-DFS production rate with lighter models, I'd say that we wouldn't be able to build enough of them quick enough without sacrificing too many production lines.