Rugged Suppressors .338 Caliber Shim Kit 6.8mm. 338 precision shim kits are designed for 3/424 M18x1 and M18x1.5 barrel threads to align your muzzle brake or ash hider. There are 3 sizes of shims in the bag. Specifications for Rugged Suppressors .338 Caliber Shim Kit: Manufacturer: Rugged Suppressors Inner Diameter: 0.76 in Outer Diameter: 0.875 in Threads per Inch: M18x1 M18x1.5 3/4x24 Projectile Caliber: .338 Caliber Package Contents: Rugged Suppressors .338 Caliber Shim Kit Rugged Suppressors SA017: .338 Caliber Shim Kit 6.8mm