Lyman MSR Ammo Checkers are machined from solid blocks of 6061 T6 aircraft grade aluminum and cut with custom tooling to minimum chamber dimensions. Their black anodized finish matches your favorite "black" rifle. Check the quality and consistency of both loaded rounds and purchased ammo. A must for any AR shooter. You can't afford mis-feeds, so get the easiest "Go" Gauge available for modern sporting rifle calibers. Features: Covers 223/5.56, 6.5 Grendel, 6.8 Rem. SPC, 7.62x39, 300AAC Blackout, 450 Bushmaster, 458 SOCOM and 50 Beowulf CNC machined to minimum chamber specs Confirm ammo will chamber smoothly