When unloading a semi-automatic firearm with a detachable magazine always do what?

First and foremost always point the muzzle in a safe direction. Then drop the magazine out of the rifle.At this point pull back on the charging handle(or bolt knob)and pull the bolt back to unload a possible loaded round out of the chamber. Insure the rifle is empty and release the bolt to close on a empty chamber before storing the rifle in its case. I also release the trigger to keep the spring tension off the bolt spring.

Much the same process for a semi-auto pistol. Muzzle pointed in a safe direction, safety 'on' if so equipped, remove the magazine, still pointed in a safe direction - turn safety 'off', rack the slide to remove any round in the chamber. Rack it again to make sure, pull trigger to release tension.