As you make your way through the missions you will start acquiring a lot of Astartes weapons in Warhammer 40K Space Marine 2. You will get to wield these weapons through the Ultramarines Operations campaign and the best armaments in the game will make it much easier completing some of the harder challenges.
While some weapons are specific to a particular class, others, like the Bolt Pistol can be wielded by almost every class in the game.
Here is a guide on how you will be able to unlock as well as equip new weapons and armor in Space Marine 2.
How to unlock new weapons and armor in Space Marine 2

When you are playing co-op campaign, you will be able to automatically unlock weapons for each of the classes. However, these weapons will start in their base forms and you will be able to gain more advanced versions of them as you make your way through the missions and earn more Weapon XP.
Armor sets, on the other hand, can be obtained by just completing a mission. Regardless of the difficulty of the quest, once you unlock the armor set you will automatically unlock its individual armor pieces as well.
Once you acquire the base weapons of a class, you will then be able to upgrade it and unlock additional Perks and Mastery Points.
How to upgrade weapons in Space Marine 2
In order to upgrade weapon tiers in the game you will need to spend a currency known as Armory Data. You will get this resource, by completing missions and Class Trials and they come in three different rarity types as well.

Unlocking higher upgrades of weapons will also unlock more perks for them. However, you will need to spend Mastery Points to upgrade, and you will get to farm this resource by playing the game and leveling your chosen weapon.
How to equip weapons in Space Marine 2
As you start campaign mode, you will be able to pick up and use each of the Astrates weapons that you can find throughout the levels. Make sure that you have completed the first mission called Skyfire, which will then let you choose a loadout from the Armouring Hall. Here is where you will get to equip new gear and mix and match your build.
However, do keep in mind that some weapons will have different unlock criteria. You will be able to view each of the requirements when choosing a loadout.
Along with unlocking new weapons, you will also get your hands on new skins and Astartes customization options when completing the various missions and chapters.