Carlson's aluminum snap caps have a spring-loaded primer plunger which prevents firing pin damage during dry-firing exercises. Treat the wool chamber mop with your favorite gun oil to prevent rusty chambers during storage. This 20 gauge model comes in a pack of 2. These aluminum snap caps have a spring-loaded primer plunger which prevents firing pin damage during dry-firing exercises. Treat the wool chamber mop with you favorite gun oil to prevent rusty chambers during storage.