Understand Gunsmith before you modify your weapon

Attachments, Apex, and ways to compare loadouts.

Gunsmith is available in Multiplayer and inside the FOB in DMZ. The Apex attachment is separate from the normal slots and can block certain attachment types. In the game, Gunny suggests loadouts from the parts you have unlocked.

Steps

Set your goal: easier control, faster movement, or a longer-range fight.
Check the available slots and Apex compatibility before you fit the rest of the attachments.
Change one part at a time, and compare the result in the training range that is available.
Do not rely on a beta loadout without checking that its attachments are available at launch.

Hint

A loadout you can control consistently matters more than a recommendation that does not fit how you play.

Summary and how to try it