In levelling up a Weapons Platforms, you unlock two types of attachments:
The first is for the specific Weapons Platform – Receivers, Barrels, Stocks, Rear Grips, and Magazines. These are sharable within the Platform across all Receivers.
The second are attachments that can be used across ALL weapons that are able to equip them. These are Optics, Ammunition, Underbarrels and Foregrips, and Muzzles. This lets allows you to collect an arsenal of attachments that only need to be unlocked once.
For example, let’s punch into the M4’s Progression track: at Weapon Level 1 and 2, you unlock the SZ Reflex Optic and the Echoline GS-X Suppressor Muzzle attachment.
Once these are unlocked, these attachments can be used across ANY weapon in the game.
So, there’s no need to unlock the SZ Reflex Optic or Echoline GS-X Suppressor 40 different times – you only need to unlock them once through the M4 Progression track to make them available for all weapons in Modern Warfare. Is there another Optic or Muzzle attachment you like? It may be on a different weapon’s Progression track – check around the Gunsmith, use that weapon to unlock them, then feel free to use it on your M4 or any other weapon.
Weapon Levels 3 and 4, meanwhile, contain attachment unlocks that are specific to this Weapons Platform: the 11.5” T-H4 Barrel and the 45-Round Mag. These Platform-specific attachments can be used across all weapons in the M4/Tempus Armaments Weapon Platform.
There is no need to re-unlock these kind of attachments for each individual Weapon in the Platform, but different Weapons Platforms have their own Barrels and Magazines to unlock.