What is Carpenter No. 158 Bolt Steel?

Carpenter No. 158 steel is the mil-spec steel used for fabricating the bolt itself. This type of steel provides the best performance and lifespan for the BCG. C158 is case- (or surface-) hardened. This means the exterior of the steel is made to be very hard, preventing cracking and surface wear. Inside, the steel is softer and more flexible, preventing deformation and stress fractures.

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