There are three possible solution in my mind:
First:
Blunderbusses Infantry 3/2-1 30cm 4 6+ - 3 90
These units shoot once at ranges up to 30cm and twice at ranges up to 15cm. A unit of three stands would therefore shoot 6 Attacks at a target within 15cm and 3 Attacks at a target at greater ranges. Units may shoot at enemies that charge them - in which case they shoot 1 Attack per stand regardless of range.
Still for 90 Points because of the 4 Hits.
Second:
Blunderbusses Infantry 3/1 30cm 4 6+ - 3 90
A handgun shot can pierce armour far more easily than an arrow or a crossbow bolt. Therefore, count enemy Armour values as one worse than normal when shot by a handgun. So an armour value of 3+ counts as 4+, 4+ as 5+, and 5+ as 6+, whilst an Armour value of 6+ can't save against a handgun.
Just because of the amount of metal that is flying in your direction.
Third:
Blunderbusses Infantry 3/1 20cm 4 6+ - 3 90
When rolling for drive backs as a result of casualties inflicted by hand gunners, add +1 additional dice. For example, if three casualties are inflicted roll four dice.
This is the Ancients Handgunners rule and it is quite fitting because of the reduced range and the loss of armor piercing but gaining the possibility of confused and drive back.
