The 355mm HGS-55 is a weapon attachment featured in Call of Duty: Modern Warfare 3 (2022) and Call of Duty: Warzone 2.
It is one of the Barrel attachments available in MW3.
The 355mm HGS-55 is a Weapons Platform Specific Attachment, which means it is only usable within its specific Weapons Platform.
Here's how to unlock the 355mm HGS-55 attachments in COD Modern Warfare 3 and COD Warzone 2: Get the SAKIN MG38 to Level 2.
Here you can find the list of Weapon Blueprints that include the 355mm HGS-55 attachment in COD Modern Warfare 3 and COD Warzone 2: