Thread size 5/16-27 Fits 20 & 28 Ga. The Birchwood Casey 20 Gauge Casey Gun Cleaning Mop is specifically designed for cleaning shotguns with a 20 gauge caliber. With an 8-32 thread size, this bore mop effectively removes buildup and residue from the bore of your firearm while ensuring a proper fit for optimal cleaning efficiency. Made by trusted brand Birchwood Casey, this product is essential for maintaining the performance and longevity of your shotgun.