Real life procedure is Alert, Ammunition, Range, Description and Execution. AI reaction is the important part, if player part cannot be implemented, AI reactions to seeing and not seeing the target are the least part that should be considered, because it is the part that can cause confusion and/or frustration to players.

ALERT: Alert the crewmember to an incoming order
AMMUNITION: Tells the gunner to select type of weapon and engage with that only
DIRECTION AND RANGE: Direction and range, as already ingame.
DESCRIPTION: Type of target
EXECUTION: What to do once the target is aquired, should be FIRE.

AI reactions:

ALERT: AI member selected drops previous target
AMMUNITION: AI selects desired weapon and does not switch until target is dropped or destroyed
DIRECTION AND RANGE, DESCRIPTION: As is, AI slews gun on target. This is the same as current LOCK ON mode, but the AI gives an audio cue wether or not they have seen and identified the target.
EXECUTION: Player can order to fire immediately or wait. AI holds order in memory and reacts as follows, For example:

PLAYER: Gunner, Cannon, direction 250 - 1000 meters, T-100 (AI slews on target and does visibility check.)
AI: "(Visibility check passed, AI is aware of target and has clear LOS): Identified!" OR if target is not seen:
AI: "(Visibility check not passed, AI is not aware of targets location after slewing to the right direction.) Not seen!"

IF the AI does NOT see the target, player can move the vehicle around until the AI sees the target. Once the AI DOES see the target, it announces "Identified"
Player can now give the execution command, or if it has already been given, AI announces:

AI: "On the way!" and fires the shot. If player has held fire command, AI does not announce but fires as soon as command is given. This can be omitted because not technically part of the problematic procedure.

Further example AI command:

Player: "Gunner, MG, direction 055 - 100 meters, soldiers, fire!"
AI: (Slews to target direction and performs check to see.) "Identified, on the way!"
(AI engages until all targets are destroyed OR player gives "Cease fire" order. Dropping or killing target does not stop AI to engage against infantry.