Carlson Snap Caps 28ga prevent damage and prolong main spring life. Snap Caps are a critical training aid for learning how to fix malfunctions quickly and efficiently not only for competition shooters but for anyone who has a gun for self-defense. Aluminum won't damage the chamber/extractor/ejector. Primer material is correct hardness to prevent striker/firing pin over-protrusion. They are also precision machined to true SAAMI specs.