A question. Would you simply ignore pistol vs rifle when determining damage? Because, despite usually being smaller caliber, rifle rounds are typically far more damaging than pistol rounds simply due to muzzle velocity.
Your earlier picture shows that. A 9 mm pistol round scratches the paint while your 5.56 mm rifle round punches straight through.
I would. For a few reasons:
1. Velocity does impacts wound channel, but mostly penetration and range. You could take less tissue damage from a high velocity round that went right through you, then you would from a lower velocity round that mushroomed and stayed inside you. In fact, that's why they created rounds that are designed to tumble in the first place, and why organizations like the FBI demand the rounds they use penetrate only a certain distance. For maximum stopping effect.
2. Because of so many factors, then you go down that dangerous path I mentioned earlier, about getting too complicated and bogging the game down. There are so many different ammunition types that you'd be all over the place trying to capture them all. For example, a .357 is used both in pistols and rifles, and acts much differently than a similar sized round in the 9mm. But are you using JHP? FMJ? Frangible? High Velocity? And if the mass and velocity are high enough, even if it goes right through you, it could still pretty much wreck you. Just way too many things to factor and way too many lines to draw. I've been down that rabbit hole. Often. And keep getting sucked back in
3. Since velocity impacts penetrating power and range much more than wound channel (even though it has a big impact here as well), the difference in captured better in game in the much different ranges for each (the max eff range for a pistol is about 50m while a rifle is 400ish. I.e., a medium pistol might do the same damage as a medium rifle at each bullet's effective range, but you wouldn't see it used nearly as often because it has a crappy effective range. I.e, getting hit in the chest by a 50 AE (.50 cal Desert Eagle) at 30 yards and a 7.62 at 200 yards isn't gonna matter too much to me which was worse. I'm pretty much screwed.*
So yeah, it comes down to a point where you just have to bring back a notch and go with a simpler approach knowing that there will be cases where it might be off.
But less than being beheaded. With a sword.
I imagine beheading would be considered a critical hit, and doesn't even exist in D&D rules unless you're using a vorpal weapon. The equivalent in game mechanic for a gun would probably do the same thing. I mean, does losing your whole head in a clean slice really make much of a difference compared to losing half of your head from a 7.62 round? And you're comparing slashing vs piercing. It would be like saying "Well, I can remove a head with a perfectly placed hand axe, but can't with a perfectly fired arrow, so that means the hand ax should do much more damage than the arrow." or "A spear head is three times larger than an arrowhead, so it should easily do three times the damage." Weapons do different types of trauma, so rather than look at what it could do with a perfect blow, we look at what it typically does in average combat. Ultimately, realism goes out the window for ease of gameplay, survive ability, and fun. It's just something we have to accept. It's why I think it's flawed to introduce firearms into the game and try to emulate realism there, when we don't with the weapons that already exist in the game.
*To avoid confusion, do not equate a .50 AE round with a .50 cal rifle round. Completely different. Caliber is determined by projectile diameter, not mass or velocity. It's like how a .22 and a .223 have almost the exact same diameter, but one is a tiny little plinky varmint round, and the other is used as the NATO standard rifle round (5.56). One is stopped by a small book, the other will go through 1/4" of steel plate with ease.
