Weapon perks are added through the weapon mods menu. Since update 3.9, the important distinction is whether a perk needs the weapon in your hands or remains active when it is holstered.
Read the icon before choosing
A blue hand marks an equipped perk: it works while you hold that item. A green holster marks a persistent perk: it functions as a passive effect. These categories explain why swapping weapons can change a build's behavior. Read the individual effect too; the icon alone does not describe its trigger or strength.
Weapon levels no longer gate mods
Update 3.4 disconnected weapon mods from weapon-level requirements. Mods instead use their listed in-game cash or C-stack cost. This does not mean every mod is free. Update 3.9 also made some previously exclusive mods shareable across more weapons; it did not make every attachment compatible with every gun.
Compare at the shooting range
Open the main Play button and choose the shooting range. The official description supports switching loadouts and testing targets at different ranges. For a useful editorial comparison, keep distance, target and aim point consistent; change one perk or mod at a time. Record your settings before drawing conclusions. A result on a stationary target is not a universal damage ranking for every enemy or difficulty.
Check the patch before copying a combo
Update 3.10.1 fixed the Thunder’s Scythe Quickshot and Return Policy reload conflict and renamed the Lawman's Quickshot perk to Lethal Cadence. It also corrected ARGES damage-reduction decay while firing. An older build explanation may describe a bug or name that no longer applies. Verify the current menu rather than searching indefinitely for the old label.
Sources
Based on official documentation. Gameplay advice is editorial unless a test is explicitly documented.