This .221 Remington Fireball Modified Case is specially engineered to work with Lock-N-Load Precision gauges for accurate reloading measurements and chamber alignment..221 Remington Fireball Modified CaseSpecial fine thread attachment for Lock-N-Load Precision gaugesEnsures perfectly square alignment in firearm chamberModified neck made perfectly round for precise bullet seatingSlightly opened case mouth allows standard diameter bullets to slip in concentricallyEssential for precision reloaders seeking the most accurate measurements for optimal cartridge performance and consistency in .221 Remington Fireball chambered firearms.