Which components are most critical to lubricate on an M16/M4 during routine maintenance?

• A. Bolt, bolt carrier, cam pin, gas rings, and the rails or contact points inside the upper receiver.
• B. Trigger, hammer, magazine follower, pistol grip.
• C. Stock, handguard, optic mount, front sight post.
• D. Barrel exterior, bayonet lug, flash hider, muzzle nut.

Answer: (A)

Lubrication is most critical on the moving parts that slide and seal during cycling. The bolt, bolt carrier, cam pin, gas rings, and the rails or contact points inside the upper receiver are the surfaces that experience the most friction as the firearm fires and the bolt carrier cycles. A light film of lubricant on these parts helps the bolt and carrier slide smoothly, reduces wear, and keeps the gas-sealing surfaces working reliably. The other components listed—trigger and hammer group, magazine follower, pistol grip, as well as stock, handguard, optic mount, and front sight post—are either not involved in the primary sliding action, or are better kept relatively dry or minimally lubricated to avoid attracting debris or interfering with safe operation. Exterior barrel surfaces and muzzle devices aren’t about sliding friction during normal cycling, so they’re cleaned and wiped rather than heavily lubricated. In short, focus lubrication on the bolt, bolt carrier, cam pin, gas rings, and the internal rails where they contact the upper receiver.