Chamfer
Cutting a small inside bevel on a trimmed case mouth so a bullet starts straight and seats without shaving its jacket.
After a case is trimmed, the inside edge of the mouth is left square and often slightly burred. A chamfer tool cuts a shallow funnel into that inside edge so the base of the bullet finds the case mouth cleanly and slides in without scraping copper off the jacket.
Chamfering pairs with deburring, which cleans the outside edge. Together they are the finish work that follows case trimming. A light, consistent chamfer helps every bullet enter on the same axis, which protects concentricity and therefore accuracy.