The Neck Die Bushing fits in the Neck Die and sizes the case neck to grip the bullet. To calculate bushing size... Measure the diameter of a loaded round where the bullet sits in the neck (Ex. .268). Now subtract .002 to .003 from that number. .268 -.002 = .266 Bushing Another way is to measure your wall thickness and multiply by 2 and add bullet diameter. Then subtract .002 to .003 from that number. .012 Wall Thickness x 2 = .024 .243 Bullet Diameter Example: .243 + .024 = .267 Now subtract .003 (.267 - .003 = .264 Bushing)