Material: Bushing holding body: 303 Stainless steel. Headspace & bullet comparator bushings: 6061 Aircraft grade aluminum alloy. Surface color-coded and hard anodized with a ceramic-like wear-resistant surface. Compatible caliper jaw thickness: ≤4.70mm Bullet bushing sizes: .17, 20, 22, 24, 25, 26, 27, 28, 30, 33, 35, 37, 41, 45 Headspace busing size: .330, 350, 375, 400, 420