Ammo straps are items that can be worn across the chest and are designed to increase the player’s ammo capacity and reloading speed in the game. There are three types of ammo straps available: the Ammo Straps, the Bullets Ammo Strap, and the Shells Ammo Strap.
The Ammo Straps and the Bullets Ammo Strap can be worn as standalone clothing accessories, while the Ammo Straps (weapon part) can be attached to certain weapons as an upgrade using a screwdriver. It is unclear if the ammo straps do indeed increase reload speed as claimed, but it is thought that they may offer a very small increase.
The Shells Ammo Strap, on the other hand, cannot be used as an upgrade for a firearm and cannot be attached to it. It is also possible that the Bullets Ammo Strap is “bugged,” which may prevent it from functioning as intended.
To attach an ammo strap, simply wear the Bullets Ammo Strap or Shells Ammo Strap as an accessory, or use a screwdriver to attach the Ammo Straps (weapon part) to a compatible weapon. Keep in mind that the effectiveness of the ammo strap in increasing reload speed is still up for debate.