I've played the modern mod, as well as a very similar but less balanced and elaborate mod I made of my own but didn't release for adventure mode. While it's fun for a short time, I run into the same problems.
The main issue is the rate of fire, which is so slow that unarmed opponents or yourself can pretty much cover the distance on a flat plain from encountering your opponent to touching them in two to three shots if you're lucky.
Bullets also were a huge problem in my mod, as giving them enough power to do the damage a real bullet would required so much force that full sized men would be flung out of the line of sight with more powerful weapons. The modern mod had a much better balance of realism between force and damage than mine, but I still see the issue even in that modern weapon mod.
Also, the AI and current game mechanics, like ambushes, really sucks at dealing with the lethality of firearms. Typically, ambushes even on my very high level dodger character result in me escaping badly wounded about half the time and blown to pieces before I can react the rest of the time.
The questions I have for you more experienced modders:
1. Has anyone figured out ways or tools to modify the game in a such a way that rate of fire isn't super slow and entirely dependent upon the user of the weapon?
2. Has anyone made bullets that behave more like real bullets with very high speeds and penetration but without requiring an absurd amount of force to achieve this?
3. Is it currently possible to fix the AI and game mechanics to improve combat?