Complex sentence: The bullet that is to be gas checked must be designed for a gas check; there will be a rebated area to allow the cup to fit the base, or a small projection to allow attachment of the check.
Split into simpler sentences: The bullet that is to be gas checked must be designed for a gas check. If the bullet is designed for a gas check, there will be a rebated area to allow the cup to fit onto the base, or a small projection to allow attachment of the check.