This mod makes the game really challenging. It's kinda like hardcore gamemode playing with this little change to Modern Firearms mod. You will now find the most powerful weapons provided by MF from the moment you enter the wasteland, held by mischivous creatures which like firing them in your direction.
The game is much harder. Try it only if you have played the game through at least once, it may and will ruin game experience, because yoou will have a greater chance of getting beaten in a firefight where the others had disproportionatele more firepower, especially, when you just started exploring the post-apocaliptic land, and you have the 10mm pistol only, as a weapon effective from distance too.

Nothing else changed. If some NPC would spawn with a shotgun in its hands, it will spawn with it. The only thing changed is the shotgun will not neccessarily be a one with approppriate power for your level. It will be any of the shotguns the MF mod contains.

If you decide to give it a try, you need to get Modern Firearms mod to re-add its weapons to the NPCs weaponry, which means disabling and enabling it again. The weapons you had previously will be gone after this; you need to collect them again.
1. Create a save of your current game.
2. Disable Modern Firearms mod, enter the game, and create another save (the game will note that the Modern Firearms.esp won't be loaded)
3. Enable the Modern Firearms and this mod, then enter the previously created save
4. A message will appear telling that Modern Firearms are sold across the wasteland (takes a few seconds to appear)
That's all. The weapons are added to the NPCs possible weapons list, with no level requirements.