Almost every AA piece was used to fire on infantry. The 40mm Bofors is widely used in game (as it was historically). This gun fires a 40mm explosive shell at a rate of 130 per minute. Without question an infantry formation attempting to advance in the face of such extreme firepower would be wiped out if it could be brought to bear. In reality this was rarely the case as the crew is typically scanning the skies and the noise of the gun along with its high profile allows for easy detection and covert approach by infantry across all but the most open and flat of grounds. At mid/close range the crew taken unawares is desperately vulnerable. None of these variants have an AT mode because the Bofors was not used to fire on tanks as that would be an absolutely ridiculous misuse of the weapon and ineffective against all but the earliest AFVs.

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While the crew is exposed, as this picture shows placing one in an entrenched positionwas entirely possible.
The GAZ AAA 4M by contrast is equipped with four Maxim MMG with a 7.62mm x54, a rifle caliber round. It is an unarmored pickup truck with wooden slat sides with no cover for the gunner whatsoever. Due to the nature of trucks it is typically difficult to place one in a fortified position without compromising the mobility that a vehicle mounting affords. Also as a converted civilian truck there's really only so much you can do with the dang thing.
Here is a picture of one:

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A crew of this vehicle that decided to fire on infantry rather than running for cover or making a hasty escape would quickly find that this poor decision was their last. Engaging even the earliest and least capable of AFVs with one would be an act of breathtaking stupidity.
So while it may have fired on infantry, it does not need a special mode for doing so and absolutely should not be used in that role any more than other towed or vehicle mounted AA guns should be. The sole exception to this rule is the venerable meat chopper which had an armored body, a protected gunner position (in firing position the gunner can barely be seen) and a breathtaking rate of fire. It was routinely used for infantry support when the enemy lacked the means to drive it away and my goodness did it ever perform.

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