Bronze brushes are made with military specification phosphor bronze filament and are available in 16 different calibres or gauges from .17 to 8 bore. Larger calibres have oval core wires for strength and durability. Larger caliber have oval core wires for higher resilience and durability.Phosphor bronze is an alloy that is used when high density and strength is needed. With this material, you have the best quality you could wish for durable and dimensionally stable wire brushes.