Just for fun, I used the Damage = 4 * Log5(Kinetic Energy) conversion on a longer list of cartridges than I provided here.
As you can see, most weapons fit into a fairly small range of stats. The .22LR does about as much damage as a shortbow (between 1d6+3 and 1d6+4), where a 7.62 Battle Rifle is about 2d10, which converts to 1d10+5 or 1d10+6 – about as damaging as a longsword held in both hands by a ST 20 fighter.
I like using multiple dice because they both roll if you crit, which gives a lot of room for very serious wounds.
That’s the other thing that could potentially be done here, as well, using the principles from expanding the crit range for different armor types. Larger and more damaging projectiles could (should?) have a wider range where a hit is also a crit. I based this entirely off of the mass of the bullet, using a logarithmic scale. My first attempt gave 5/4 * log (Mass) for the width of the range (a .22LR was 2), which gave 19-20 for the anemic .22, and 17-20 for the mighty .50BMG. A 25% chance of a critical hit is pretty great, so I stopped there, though obviously it could be tweaked even more.