Gas Check
A small metal cup crimped onto a cast bullet's base that shields the soft lead from hot propellant gas at higher velocity.
A gas check is a thin copper or gilding-metal cup that snaps onto a ridge at the base of a cast-bullet, forming a tough cap over the soft lead. Its job is to protect the bullet-base from the searing propellant gas behind it, which would otherwise melt and smear the rear of a plain lead bullet against the bore. The check seals the base, helps the bullet obturate to fill the rifling, and is sized in place when the bullet is run through a sizing die.
The practical payoff is a higher usable velocity before leading begins. A plain cast bullet may foul the bore badly when pushed hard, while a gas-checked version can run faster and cleaner because the lead never sees the flame directly. That extra margin against fouling is why rifle cast bullets intended for full-power loads almost always wear a gas check, while light target loads often do without one.