Based on comments from the previous post, here are my revised Home Rules. The basic change is that I'm now using a formula to determine the KE brackets for each die of damage. Size of damage bracket in KE = Cumulative sum (Damage Dice) *64. So the first damage bracket is Cumulative Sum (1) * 64 = 64 KE. As I said in the previous post, I centered it on the KE of an clothyard shaft (64); so the range is 33-96. For the second damage bracket it is (Cumulative Sum (1,2))*64 = 128; third bracket range is (Cumulative Sum (1,2,3))* 64 or 384. This has the effect of slowing the damage progression, modeling the ability of firearm projectiles to penetrate completely through the body without transferring all of their KE to the victim.
|KE||Base Damage||Damage Range
|0-96: 1 die damage|
Arrow(Clothyard shaft) 
Handgonne - medium 
Handgonne - short 
|97-224: 2 dice damage|
Prussian Dryese Needle Gun 
Wheellock Pistol 
1851 Colt 'Navy' Revolver 
Flintlock Pistol (Thompson)
|225-480: 3 dice damage|
Matchlock Musket 
Colt .45 M11911A1 
|481-928: 4 dice damage|
|929-1632: 5 dice damage|
Flintlock Musket (Brown Bess) 
|1633-2656: 6 dice damage||1633-1974||5d6+1d4||6-20-34|
|2657-4064: 7 dice damage|
M1 Garand 
|4065-5920: 8 dice damage||4065-4683||7d6+1d4||8-27-46|
|5921-8288: 9 dice damage||5921-6710||8d6+1d4||9-31.5-52|
|8289-11232: 10 dice damage||8289-9270||9d6+1d4||10-35-58|
|11233-14816: 11 dice damage|
M2 Browning .50 MG [13,166]
Firearms are ranged weapons and attack accordingly. The following special rule is used to model grazing shots. Attack rolls between Target AC -2 and Target AC +2 are grazing shots and do 1d3 point of damage. Attack rolls greater than Target AC +2 do full damage. For example, if the target in chainmail (AC 16) is shot at with a Wheellock pistol, attacks of 14-18 will graze the target and do 1d3 points of damage. Attacks greater than 17 would do 1d6+1d4 damage. A natural 20 always does full damage, regardless of the target AC.
Critical Hit Multiplier
Pistols and Handgonnes have a multiplier of 3; rifles and muskets have a multiplier of 4.
Massive Damage for all systems, I'll use a variation of the d20 Massive Damage rule. If the damage exceeds 50 points (and you're still alive) Fortitude save DC (15+ damage/10) or die.
Previous Firearms posts
Firearms in Fantasy Campaigns III - Damage
Firearms in Fantasy Campaigns II - Technology by Era
Firearms in Fantasy Campaigns I