Before you start scrubbing, it helps to understand what you're working with. The bolt carrier group consists of several discrete parts, each with a specific job and specific failure modes. Bolt Carrier The outer body that houses the bolt and rides inside the upper receiver. During cycling, expanding gas enters the gas key and drives the carrier rearward. The carrier key (also called the gas key) must be properly staked to prevent gas leaks. The inside of the carrier, especially the firing pin c …