A barrel shim is a thin piece of material that is used to fill in a gap or space between two components. In the context of firearms, a barrel shim is often used to adjust the alignment or fit of a barrel in relation to the upper receiver or handguard of a gun.
For example, if the barrel of a gun is not properly aligned with the upper receiver, it can cause the barrel to shift or move when the gun is fired, which can affect accuracy. A barrel shim can be used to help correct this misalignment by filling in the gap between the barrel and upper receiver, which helps to stabilize the barrel and improve accuracy.
Barrel shims are also sometimes used to help install or remove barrels from a gun, particularly if the barrel is tightly fitted or if the barrel nut (a component that holds the barrel in place) is difficult to remove. The barrel shim can be used to apply pressure to the barrel and help loosen it from the barrel nut.
Barrel shims are typically made of metal or a durable plastic material and are designed to be thin enough to fit into small gaps without affecting the overall fit or function of the gun.