What is the system of operation for the M4/M16?

• A. Gas direct, select fire
• B. Gas piston, select fire
• C. Recoil operated
• D. Lever action

Answer: (A)

The system being tested is a gas-operated, direct impingement action with select-fire capability. In this design, high-pressure gas from the fired cartridge is tapped from the barrel through a gas port into a gas block and then directed back into the receiver to push the bolt carrier rearward. That rearward travel cycles the action: it unlocks the bolt, extracts and ejects the spent case, strips a fresh round from the magazine, and then returns the bolt forward to lock again. This direct transfer of gas to move the bolt carrier is what defines direct impingement, as opposed to using a separate piston or relying on recoil alone. The “select fire” aspect means the firearm can be switched between different firing modes (semi-automatic and a burst or full-automatic setting depending on the variant). The other options don’t fit because a gas piston would employ a separate moving piston, recoil operation uses different mechanical timing tied to the barrel’s movement, and lever action is a completely different mechanism.