What must you do after inserting the barrel into the receiver but before closing any covers?

Inserting the barrel into the receiver is a critical step in the assembly of the firearm, and aligning the gas cylinder is essential before proceeding further. Proper alignment of the gas cylinder ensures that the gas system operates correctly, which is crucial for the functioning of the weapon. If the gas cylinder is not aligned properly, it can lead to malfunctions or reduced performance once the firearm is fired.

Performing this alignment step before closing any covers is important because once components are closed or secured, access to the gas system is restricted, making it difficult to make necessary adjustments. Thus, ensuring the gas cylinder's alignment at this stage is vital for the safe and effective operation of the firearm.

Checking the safety, inspecting the chamber, and clearing the ammunition are important steps in firearm handling and maintenance, but aligning the gas cylinder is the specific requirement following the insertion of the barrel into the receiver.