Activision puts it plainly: “In Modern Warfare 4, each Attachment is unlocked only once, meaning that once unlocked, the same Attachment will be available across all weapons that use it.” Every other decision about attachments follows from that.
Unlocked once, works everywhere
A stock you unlock while levelling the M4 is not a M4 attachment. It is a stock. The moment you unlock it, it is available on every weapon that accepts a Stock slot, and it never needs unlocking again. That makes the attachment pool a shared resource across your whole arsenal rather than a set of per-weapon chores, and it means a low-level player with a wide attachment pool can build a competitive class on a weapon they have barely levelled. Activision also changed what levelling costs: "Weapon Progression Speed: If you reach a level on a weapon that would award you an Attachment you already own, you skip that level with a Bonus Level Up." Every attachment already in your pool shortens the next weapon you max.
What the five stats actually are
Gunsmith shows five values when you add an Attachment or compare two weapons: Damage, Range, Accuracy, Mobility and Handling. Activision notes you can go deeper into weapon statistics through the Details menu. The important part is that they trade against each other, so a build that pushes Damage usually costs you Mobility or Handling, and the Firing Range is the only honest way to feel that.
What each slot does
Attachments are grouped into slot types, and each one changes a different part of how the weapon behaves. The table below is Activision's own description of the slot types. The guide describes ten types here, while the per-weapon Attachment Types list also names weapon-specific slots such as Trigger Action, Bolt, Guard and Cylinder. A slot with no standard type has its own behaviour, so those names show up in the weapon tables rather than in this list.
| SLOT | WHAT IT CHANGES |
|---|---|
| Optics | Improve target acquisition with a variety of sights, helping you line up shots more effectively. |
| Lasers | Tighten hip-fire spread, support a Canted Stance, highlight enemies, and provide other close-quarters aiming benefits, though some lasers may reveal your position. |
| Barrels | Adjust a Weapon’s damage range, bullet velocity, and other ballistic characteristics. |
| Muzzles | Reduce recoil, suppress weapon fire, or otherwise refine shot control and weapon accuracy. |
| Underbarrels | Enhance recoil control and aiming stability or equip specialized launcher attachments where available. |
| Stocks | Boost mobility, accuracy, flinch resistance, or handling related handling traits. |
| Rear Grips | Adjust weapon handling, including Aim Down Sight (ADS) and Sprint-to-Fire speeds. |
| Comb | Adjust handling statistics including ADS and Sprint-to-Fire speeds, Flinch Resistance, and ADS Idle Sway. |
| Ammunition | Modify your weapon’s rounds to alter ballistic performance, adding specialized effects or enhancing attributes like bullet velocity, damage range, or penetration. |
| Magazines | Increase ammunition capacity or improve reload efficiency to keep you in the fight longer. |
Apex Attachments are a different system
There are 28 Apex Attachments in the launch arsenal, and they do not behave like standard attachments. Activision states it directly: "Rather than offering incremental stat adjustments, Apex Attachments fundamentally change how a weapon performs. Some transform familiar platforms into entirely new combat roles, while others introduce alternate fire modes, integrated equipment, or specialized tactical capabilities unavailable through standard Gunsmith customization." They are equipped separately, they do not consume a standard attachment slot, and you unlock one by levelling that weapon to maximum.
Gunny builds it for you
Gunny is available in Create-a-Class and in the DMZ FOB Gunsmith. It uses the Attachments you have already unlocked to suggest weapon builds, and with a single input it cycles through close-range, balanced and long-range options. Activision notes those builds "evolve naturally as new Attachments are unlocked," so Gunny gets more useful the wider your attachment pool gets.
The Firing Range is the real test
There are two Firing Ranges, one in Multiplayer and one in the DMZ FOB, and you can visit either at any time to compare builds and test attachments before you deploy. The Multiplayer range tracks targets killed, accuracy, damage, moving target range and time to kill in the corner of the screen, and you can cycle through your loadouts without leaving it.
SEE EVERY APEX ATTACHMENT ↗ COMPARE ATTACHMENT SLOTS ↗ WHICH WEAPON TO LEVEL FIRST ↗
Where this comes from
Reviewed October 22, 2026. Quoted text is from Activision’s own published material. Advice that is this site’s own judgement is labelled as such.