You will be able to choose between five different groups of weapons. Here they are:
These weapons are both quite powerful and precise. As a result, you should be using them for the most of the time, however later in the game they become less useful. It's mostly because enemy soldiers will be equipped with bulletproof vests and better weapons. Nevertheless, you should be interested in these guns, especially since you will be able to upgrade them with most of the available components (scopes, supressors...).
The weapons from this group are very light and offer a possibility of attacking enemy soldiers almost instantly. It's best to use them during the missions that will require you to make quick decisions. Submachine guns are also very useful in areas filled with a lot of small rooms and corridors. Bear in mind that you will have to decide on long bursts in order to take down most of the terrorists. Furthermore, submachine guns aren't too precise. Thankfully, in most cases you will be able to upgrade them with all of the available components. I will recommend choosing the high capacity magazine. Thanks to that, you won't run out of ammo during a fight.
HEAVY MACHINE GUNS
You should know exactly what to expect from these kinds of weapons. Heavy machine guns are very powerful, however some of these weapons can also be used to fire single shots. In that case, you might want to consider upgrading them wth a scope. You won't need the high capacity magazine, because these weapons are so powerful, that you won't have to sacrifice a whole clip in order to get rid of the nearby enemy soldiers.
Two remaining groups include SHOTGUNS and GRENADE LAUNCHERS. I wouldn't recommend using any of these weapons. They are pretty useless during the singleplayer campaign. As for the shotguns, they are very powerful, however you wouldn't have a chance to use them. In order to inflict some serious damage you would have to operate at a very small range. Most of the missions have larger corridors or outdoor areas and you'd be defensless out there. I would recommend, on the other hand, giving a shotgun to one of your teammates. Thanks to that, he will be taking care of the terrorists who have decided to surprise you or the rest of the team. You can ignore the grenade launcher. There's also an additional gun. Each member of the team can carry it. You can choose between several types of weapons, however for the whole time you should be using the main weapon. Be sure to fire single shots, whenever it's possible. Thanks to that, you won't run out of ammo during the mission.