Using a D20 is good, you can put in factors that alter shooting without ruining the dice roll.

If say all units are 12+ to hit, then you can add as modifiers:
+2 elite shooters
-2 target heavy armour
additional -2 if target is barded horses
-1 target shielded
-2 militia or other poor shooters
-4 if over half range (-2 if Longbow, X-bow perhaps)
-2 if target in skirmish order
-3 if target in woods/BUA/fog/smoke/shield spell
-2 if shooter moved
For example

or it might be add shooting class, subtract armour class, get net 10 to cause a casualty. Or whatever.

On a D6 system you couldn't have all that variation.

Mark

Yeah,
Either that or I was thinking a units Shooting Statistic could be A type of dice with a modifier. For Example some Merchant Archers may have a shooting value of d8+1, so you would roll a d8, add one to it, and that is the numbers of casualties caused. the average value will be about d8.
Then we get talking about Saves....