The Henderson Precision Case Trimmer Collet steps ensure that the case head is firmly seated in the Collet, even when there is case rim diameter variation. The tapered design ensures that the Collet securely closes on the case. Collet Fit List: Collet #5 - (.605 Dia) 300 Norma Mag, 338Lapua, 33XC, 37XCCollet #6 - (.640) .375 Cheytac .416 Hell Fire .458 Vestal