The Base slot includes the greatest variety of options, from magazine modifications to camouflages. One can further customize their primary firearm by using Standard Magazines, an Extra Magazine, Open-Tip Ammunition/Slugs, and the Veteran upgrade.
A player can choose one of each of these mods, some of which can be used by only a single class—for example, the High-Power scope can only be attached on the Sniper class. Every mod has a benefit, but some have drawbacks that should be considered before use. For example, Silencers increase accuracy and stealth, but reduce the weapon's base damage. Each modification (except Ironsights, Stock Barrel, and Standard Magazines, which you start with) must be unlocked through leveling up a class and gaining upgrades.