Opportunity Fire is one of the most important mechanics of a tactical game series. There are two types of movement. Advance Movement allows a unit/stack to move a single location. For this example the US player is issuing a Tactical Movement Order. Which allows a unit/stack to both move and fire, with half their movement allowance and a modifier to their fire combat die roll.
Before the US units can fire they would have to undergo Opportunity Fire. Infantry units have an Opportunity Fire range of up to 3 hexes. They do not need to have an order to activate to use Opportunity Fire. All that has to happen to trigger the Opportunity Fire from an infantry unit is the target be in LOS and be 3 hexes/or less from the firing unit.
Support Weapons (Grenade Launchers, Machine Guns, etc.) are a bit different. They use their normal firepower out to 3 hexes. Support Weapons can activate Opportunity Fire out to the extent of their printed range. For instance, an RPK (LMG) has a range of 11, which means it could fire Opportunity Fire out to 11 hexes at a moving enemy unit. The Support Weapon loses effectiveness the farther it away the target is. The Support Weapon loses a point from it's Infantry Firepower value for each hex beyond 3 that the Opportunity Fire occurs in.
As an example: The RPK (LMG) has a range of 11 and an Infantry Firepower of 5. In Opportunity Fire it would attack a moving enemy unit at 3 hexes with 5, at 4 hexes with a 4, at 5 hexes with a 3, at 6 hexes with a 2, at 7 hexes with a 1 and at 8 hexes with a 0. You cannot activate an attack with less than 0 value. A Russian Assault Rifle Fireteam has an Infantry Firepower of 8 and a range of 3. It cannot fire further than 3 hexes with an Opportunity Fire attack for any reason.
The graphic shows the US set to move onto the hill. The only hex that would be attacked 'normally' would be from the Russian stack in H5 to the US Fireteam in hex E3. All the other locations would be subject to a 'reduced' attack value from the Russian Support Weapons (RPK LMG's) firing.
Good Hunting.
MR