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 two.
Specifications:

Type	Snap Caps
Caliber or Gauge	20 Gauge
Rounds Per Box	2
Material	Wool w/Metal Base
Chamber