Concentricity Gauge
Also: Runout Gauge
A measuring tool that spins a loaded round on rollers to read how far the bullet runs out from true center, a marker of match-grade handloading.
A concentricity gauge cradles a loaded cartridge on rollers and bears a dial indicator against the bullet while you rotate it. The needle swing shows runout, the amount the bullet tips off the centerline of the case. A round with the bullet seated crooked enters the rifling slightly yawed, which opens groups.
Handloaders use the gauge to sort ammunition and to diagnose where runout is coming from, whether a tired sizing die, uneven neck tension, or a seating stem pushing the bullet in cocked. Low, consistent runout is one of the quiet markers of match-grade handloading.