WEAPON LEVELS GATE THE ATTACHMENTS
Platform
Which front
How it is delivered
A weapon levelled up unlocks sights, magazines and muzzles, and on the harder difficulties those are the difference between a gun that works and one that does not. It is a Helldivers 2 addition rather than something the first game had, and it is per weapon rather than per account.
Four stratagem slots decide how a mission goes far more than the primary weapon does. They are bought with requisition slips, which accumulate slowly, and the full unlock row is there because most people want the whole list rather than three specific ones.