Very interesting chart. And yes it’s enormously helpful. This kind of chart can help a great deal with accuracy, so much in fact that I’d love to put it in the appendix of my missile weapons book, if you’d be willing to share it. Do you think you could do one based on the Graz test? That gives us kind of a baseline for earlier firearms and with kind of ideal shooting conditions.
In Codex terms an average, minimally trained person shooting a “wall gun” (closest thing in the 15th Century to a musket) has a range increment of 50′, which works out to the following modifiers: +2 at 50′, 0 at 100′, -3 at 150 ft, and -4 at 200′ or greater. In terms of percentage, it means you need:
an 8 to hit on a D20 at 50′ / 15 yards (60%), a 10 at 100′ / 30 yards (50%), a 13 at 150′ / 45 yards (35%), and a 14 at 200′ / 60 yards (30%) or greater.
Based on your chart it looks like, for a musket using aimed fire it should be:
a 2 to hit on D20 under 25 yards (90%), a 5 at 50 yards (75%), a 7 (65%) at 75 yards, a 9 at 100 yards (55%), a 12 at 150 yards (40%), a 14 at 200 yards (30%) or greater (though it gradually declines from there)
… so I was clearly being a bit too conservative. The range increment should be almost doubled, and the bonus at shorter range increased.
Contrary to your assumptions, I think both the weapons and the skill would be much better prior to 1550. This is what the data seems to suggest anyway. I wish I could find a detailed article about a shutzenfest, but the assumption circa 1450 was that the average person should be able to hit a target the size of a man’s head at 50 paces, roughly the same as a yard.