Modern Firearms introduces a LOT of current real world weapons to the game, at this time mostly rifles.
There are several base 'families' of weapons including the AR15, AK, HK416/7, ACR (Magpul Masada), HK MP5, SG 550-552 and many more, all of which are completely customisable at a workbench, creating variants from suppressed CQB shorties to DMR variants of most.
Customisation is NOT possible however with only vanilla game components- you'll have to find or buy parts in the game either as single parts or by scrapping the weapons you take from your enemy. It is Idlesheeps belief (a belief shared by his team) that you shouldn't be able to build a military spec rifle from wonderglue and scrapping an iron roof.
Another point of controversy with this mod is that the damage seems very high to many players, and as it features full level list integration combat is either too easy or too difficult depending on which end of the AR15 you're standing at. The reasoning of Idlesheep (again backed by his team) is that being shot should hurt. Being shot twice should stop the hurt altogether.
These weapons therefore do not follow the vanilla game balance. They don't scale with level AT ALL, so the damage at level 15 will be the same as it is at level 115. (Weapons are still affected by perks however, and you may find yourself needing those at high levels.)
The balance is found in your enemy also using these weapons, so combat will be swift and deadly. Another feature of this mod is scripting which allows you to change tactics without using your inventory and we believe that the resulting seamless and vicious combat gameplay is a huge improvement on the vanilla game.