Keep your chamber clean! One reason shooters need a little extra room ahead of the case shoulder when a round is chambered is because grunge adds up. Firing residues effectively reduce headspace. To avoid headspace changes, clean the chamber every time the rifle comes back from the range, even if you don’t clean the barrel bore. To clean an AR-15 chamber, a .357-caliber pistol brush does fine, and then just wrap a bore patch around the bristles to finish it up. Use petroleum-based solvent.