The first thing you need to know about weapons is that not all of them are available at the start of the game. Despite what it may seem, new weapons are not purchasable, but are automatically unlocked as you rank up. Ranks can be thought of as levels of experience. There are three ways to earn experience points: by killing enemies, by finding secret items, and by completing active challenges. Luckily, you don’t have to constantly check your inventory: if a new weapon needs to be unlocked to get a new rank, the game will let you know (screenshot below).
New weapons aren’t the only rank-up reward; Marines also receive points that they can spend on upgrading their weaponry. Customizing your gear is optional, but it pays off a lot considering how effective it can become on the battlefield. Upgrades are divided into five categories: alternative fire (alternative type of shooting), attachments (attachment of additional elements, such as a silencer or an extended clip), sights (mechanical, laser, optical), drawing (colorize weapons – does not affect in any way) and fire mode (switch primary fire mode). When upgrading weapons, try to adapt to the conditions on the battlefield. For example, a rifle you use to take out enemies at long range could be upgraded with the addition of a grenade launcher and a telescopic sight, while a shotgun could benefit from a component that would reduce the recoil effect. Keep in mind that modifications are made for all weapons from this group. If, for example, you have more than one pulse rifle (regular and legendary Hicks), they will both be modified.
In total, the game has 86 small tasks divided into three categories. There are only three tasks active at the moment (each from a different category) and you can view them by pausing the game. It’s important to remember that any remaining challenges you haven’t completed yet will remain inactive and you won’t be able to flag them, even if you’ve completed what is required to describe them.
Hence, focus only on active ones and try to complete them. This should motivate you that each challenge you complete will give you 500 experience, but they can also raise your stats or give you weapons in new models.
Note. Challenges can be completed in both single player and multiplayer campaigns. Unfortunately, you can’t ignore co-op modes because there’s an entire category dedicated to playing as a Xenomorph. In the single player campaign, try to choose your missions according to the current challenge objectives. For example, if the game forces you to eliminate enemies in hand-to-hand combat, choose a mission in which you will encounter lightly armed scientists.
Source: https://guides.gamepressure.com
