I concur with Delta's analysis about treating range as reducing target size, but I think this falls apart somewhat when firing at long-range dispersed targets. I think of the dispersed target as a big area that the archer is just trying to drop an arrow into somewhere. The target is quite large, so it's not hard to hit the area. But what's important here is, how much of the target area actually has a target in it? And also, what's the effect of angle of incidence -- what you really want is the projection of the actual target area into a plane orthogonal to the arrow's direction of flight (also allowing for potential overlap/shadowing of individual targets). If (to choose a simple example) the target area is 50% real target and 50% empty space, then it's simple in your system: just apply an additional -3. So I think you're baking in some assumptions about density of the massed troops. Personally I think 50% density is a bit high, but that's probably because we have different mental images of the targets.Firstly I'll recap with the help of some illustrations. If you shoot at a target which is twice as far away, then that is the same apparent size as a target which is half the size (which is a quarter of the area):
The distribution of shots is a bivariate normal distribution (i.e. both x and y are a bell curve), so 1000 arrows shot into a target would look like this:
I don't have any armies to hand, so I was going to take photos of miniatures (Lego figures are not proportionate) but it's tricky to see as my figures are all on painted bases, so I've taken this approximation of people as x 9.5'' x 1'6'' x 5'9'' and made stacks of Lego bricks with the same proportions. I've given a gap of 2 studs - 1'6'' - which seems pretty close when you're holding a sword. According to Wikipedia this is the spacing of a tight phalanx.
Viewed head on, only the front rank can be seen:
However, from a distance, arrows will come in at an angle, so present a target something like this:
In contrast let's consider the ultra tight synaspismos formation the phalanx adopted when under "extra pressure, intense missile volleys or frontal cavalry charges". This is no spacing between figures, and only one stud between rows. At 48 degrees there is now only 112 sq ft presented to the arrows of the enemy, this might result in a small -1 to hit penalty, but overlapping shields would result in a more significant advantage.
Now a phalanx is tighter than you'd expect - if the spacing between troops was doubled as per a loose phalanx, then the troops would occupy four times the area, and when the arrow comes down onto the phalanx three quarters of the area would be empty. This would apparently give a -6 penalty to the archer, but on the other hand the troops would cover a much larger area.
For comparison, Here's loose, tight, and ultra-tight formations together in a single photo:
That is we either have loose order:
And either way, assuming the archer has less than 50% chance to hit, the troops cover a quarter of his target circle, and hence -6 is a good penalty.
Mass Targets or Individual Targets?
Leland also asked:
I guess one question is, where do you transition from shooting at an individual target, vs. at an area that is n% full of targets?Due to the logic behind how I derived mass target shooting, I'm limiting archers to a minimum of 11 needed to hit mass targets (i.e. minimum of 50% chance to hit). Hence at close range it can become better to switch to individual shooting.
The chance to hit is the best of individual shooting (with range penalty) and mass shooting (-6 fixed penalty, min 11).
For example, at 120' range a +11 archer only needs 7 to mass hit troops with a parry of 12. This is below the lower limit for mass target shooting (11), so they need an 11. For individual shooting it would be a -12 distance penalty (-3 per range category), so a 13 to hit. Hence they stick with the 11 target.
If the troops were 80' away the archer only needs a 10, so they can now switch to individual shooting.
Hitting Important Targets
A third question was:
Another thought that occurs -- you could have archers shoot at one individual target in a mass, and if they miss by, um, not very much (depending on the density and other modifiers) you could rule that they hit SOMEBODY, just not the guy they were aiming for.Now imagine that one of the targets is a special target, an important NPC perhaps. Determine the number of troops around him and roll to see whether he is targeted. E.g. if he is one of 20, a 1 on a d20 would pick him. I'd say one in a hundred maximum. If he is selected then we can see that the rest of the troops are now irrelevant and you simply make your attack roll against him. If it misses him it misses everyone. It he is not selected then you make the attack roll against the normal troops.
This same method can be used for mounted troops - fifty fifty chance of targeting the mount (depending upon the size of the mount).
Now back to considering Crossbows. My youngest is keen to to do some field experiments with his toy Crossbow...