This is what happens when you either mess up a shotgun breach, or your shotgun isn't working properly and you need to prove it to someone in the middle of an operation.

Remedial training is a form of training that happens to correct a problem. It's not usually conducted in the middle of an actual combat operation like this however. So, either this Marine massively messed something up and he was getting some extra practice in to rectify the problem, or he was having serious issues with his shotgun and needed to prove it to his squad leader or platoon commander.

Either way, as evidenced by this video, it appears that this Marines shotgun is having some issues feeding rounds. That leads me to believe he is proving an issue rather than being asked to get some remedial training in. I say this because you normally wouldn't see Marines doing this kind of thing in the middle of a combat operation, which you can clearly hear is happening if you listen to the radio chatter inside of the video.