The bolt carrier is an indispensable component of semi-automatic firearms like AR-15s and MSRs. The assembly contains the bolt, bolt carrier, and a firing pin–each of which must operate in excellent condition.
When the gun’s hammer strikes the firing pin, the pin is pushed forward and strikes the cartridge’s primer cap. The priming mixture inside the cartridge explodes, propelling the bullet out of the barrel.
The priming mixture is typically a smokeless powder, and while it’s effective at firing bullets, no primer powder burns with perfect efficiency. This means a little bit of carbon is deposited onto the inner walls of the barrel while shooting.
After multiple uses, the carbon buildup gets thicker and can become almost solid, causing the rifle’s parts to stick together, creating an unsafe and virtually inoperable firearm.