Possible Clues:

• Old fire-retardant
• Like very good rounds, will never get fired
• The way the very good rounds can't get fired?
• Like the very good ones, so it's wrong never to be fired
• Bassets milling round old condemned building material
• Fireproof material, now known to be dangerous
• As there are no better rounds, there can't be any firing
• Fibrous material, doesn't burn
• Like the best circles, no firing there
• Boss set a non-burning material up