If I activate a unit equipped with, say, a HMG, and I choose to fire the HMG, does the unit AND the HMG form a fire group? Or can I only choose to fire with either the unit OR the HMG?
When you activate a unit with a weapon, the weapon is also activated. When forming a fire group, you may use any combination of activated weapons and/or units (providing you meet the other requirements).
So if you activate a unit and its HMG, you may fire both individually (2 separate fire attacks) or fire both together as a single fire group.
In other words, firing a weapon does NOT preclude the unit that is carrying the weapon from firing.
You do activate the weapon and the unit, and they MAY form a fire group, but to not have to. The HMG could fire at one unit, and the unit another. Each could fire at the same unit in 2 separate fire attacks. Or, they could form a fire group.
If you activate the UNIT then either the unit OR the HMG can fire. If you activate a LEADER (that has the unit within its command radius) then the unit and the HMG can fire, either as a fire group or separately.