ABIW...Somewhat simplistic view.
How does one check that it is clear without touching the working parts? Assumption that it is clear is the ONLY reason people have NDs if they intentionally pull the trigger, because they ASSUME it is clear when squeezing off the action.
Making sure the chamber is clear first (by feel if necessary) is the ONLY way to avoid an ND if one chooses to pull the trigger, having pulled the working parts to the rear.
Of course there are other types of ND, such as loaded, cocked firearm, safety catch gets knocked (difficult I know) off, then rifle etc is accidentally knocked, dropped etc and it goes bang. As does one's salary.
Going back to the Rock Cpl, why did he get bounced? Unless the SAC was at an unloading bay etc, the Cpl can't supervise his entire section's weapon handling drills on the streets.
Do you chaps ever have NDs with bombs, rockets, cannon and missiles? I imagine it's a little more serious than the odd 5.56 round. How much does that cost????