Why be without spare gas rings or an extractor? A complete bolt can be at your disposal ready to be installed, and the parts necessary to repair the faulty bolt are stored handily for the needed repair. You just supply the level of spares you think you might need.  Store your CR123 batterries in the Tac-Pac instead of on your carbine.
This is the complete M4 Tac-Pac. How it works: Tac-Pac how it works video
What is it? The TAC-PAC is a modular storage system designed to store tactical batteries and other small parts. It consists of an injection-molded housing with interchangeable EVA foam die cut inserts. The housing and insert is closed with a snap-on rubber sealing lid. This is then placed into a tactical nylon storage pouch.