Since I've apparently grown computer illiterate in my old age, I have no idea how to actually make videos anymore on the PC. So, I just made SWEEET ASS MSPAINT SCREENSHOTS TO EXPLAIN!
This is the Battlelog method of changing your loadouts. You can do it much easier in game, but this way is a lot more informative.
Step 1. Go to battlelog.battlefield.com/bf4 Log in with your Origin ID.
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Step 2. Hover over the 'Soldier' button, and go down to 'Loadout'.
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Step 3. Select the weapon on the class that you'd like to change. In this case, I selected my rifle on my Recon class.
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Step 4. Choose the weapon that you prefer. Hovering over the weapon should list its stats, allowing you to choose the best weapon for what you want to do. Click the gear to modify the attachments for each weapon.
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Step 5. Choose your attachments, and click save.
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Step 6. Shoot baddies in the face with properly modified weapons.