Like previous titles in the Battlefield series, Battlefield 4 locks away a host of weapons that you can only unlock once you've accomplished certain, well, accomplishments. You can unlock these weapons, gadgets and customization features by playing through the multiplayer game and leveling up.
What's different with Battlefield 4 this time around is that a small array of weapons are locked away behind the game's single-player campaign. These include the M412 Rex, Shank, Machete, QBZ-95-1, P90, and M249, which will only unlock after you've played through the game's single-player campaign.
So before going online, it's a good choice to play through the game's single-player missions and experience the storyline that DICE put together.
Here's proof of that: