I realized I never really explained my logic for my gun damage in my previous post, so here it is.
The basic is, well pretty obvious. Damage and range both increase with the size of the gun
That's enough details, we're done. BUT what if we want a system/setting with more than 1 gun technologies going at once? There definitely were several periods in history where multiple "levels" of weapon technologies were used at once. That's complicated to achieve. Modern weapons are usually better in some way - how do we keep older weapons relevant?
So here are some of the ways you can "spread the balance" a bit.
For the matchlock to flintlock era, the way to balance those two is that the early flintlocks (the wheellocks) are quite expensive (usually nobles used them), and are almost never made for muskets. So matchlocks are much more available, and the biggest one do more damage that the biggest wheellocks. The PCs will probably try to get them.
|A double shot (?) ornate wheellock pistol. MET museum, NYC|
From flintlock to caplock. In this era, a few things are happening. First a caplock singleshot muzzleloader is not that much greater than a flintlock (effectively, it's more reliable). So all you need to do to balance those types of guns are costs. BUT there are also revolvers to consider. Revolvers are more expensive yes, but *so* much better. Here some balance is achieved by lower damage (1d6 vs 1d8), this bein because revolvers were of smaller caliber than the one shot pistols. Also, the advent of paper cartridge has made single shot muzzleloaders a bit faster than before, thus allowing them to attempt to keep up.
From caplock to metallic cartridge, to maintain balance I had to do a few things - first I'm limiting them to earlier, rimfire cartridge (black-powder) to limit their power (after a while, modern guns are just so much better). Also, the Minié ball increases the power of the cap and balls revolvers. Older single shot guns are still in the game because they too get a damage boost - the best metallic cartridge gun does a d10 dmg, while the best single shot muzzleloader does 2d8... but if you step forward a few more decades in history, the muzzleloaders are left far behind, so it's why I am stopping here.