The Legendary Extended Rifle Mag in Once Human is one of the best weapon attachments you can get for free by solving a puzzle.

Once Human has multiple weapon attachments, and they can be purchased or obtained by solving riddles on the map. The Legendary Extended Rifle Mag increases your magazine size by eight bullets while slightly increasing your reload speed. It can be compatible with any SOCR and KAM series weapon, making it flexible even when equipped with low-level weapons and maximizing its benefits.