The 3 Lug Adapter features the patented Go/No Go Gauge. The No Go/Go Gauge provides confirmation that your suppressor is properly attached, for a surprise free shooting experience. The 3 Lug Adapter is compatible and used in conjunction with OSS's 3 Lug Mount. This 3 Lug Adapter is made to 9mm HK spec and is not compatible with 45ACP.