as everyone knows, we are stuck with the Flak-38 and M1 AT guns for all nations. It doesnt make much sense an American engineer builds a German AA gun during battle.
Some reccomendations:
Battle Of Moscow
Allies
12.7mm DShK Heavy MG (with AA sight)
RoF: 600 RPM
Muzzle Velocity: 850 m/s
the DShK was mounted on a tripod to be used as an AA gun.
Ammo: 12.7×108mm
Germany can keep it’s Flak 38
Invasion of Normandy
Allies
37 mm gun M1
Stationary autocannon.
RoF: 150 Rounds/Min
Muzzle Velocity: 910 m/s
Ammo: APC-T M59A1 Shot, HE-T SD M54 Shell.
Germany used the 8.8 Flak but it may be too op as it can be spammed against troops. (unless certain depression angle limits were implemented)
Battle of Berlin
Allies
37 mm automatic air defense gun M1939 (61-K) (this can be made stationary to avoid anti-infantry spam)
RoF: 160-170 RPM
Muzzle Velocity : 880 m/s
Ammo: OR-167N (Frag-T), BR-167 or BR-167P (AP-T/HVAP), Unknown HE
If anyone else has more ideas please fell free to comment below