Why's this US Army soldier knocking his rifle magazine against his helmet? [closed]

We see soldiers do this in other movies - Apocalypse Now (during the Ride of the Valkyries attack sequence) comes to mind - particularly because this is mimicked by the marines watching the sequence in the cinema during Jarhead.

I don't know if this is really done by some soldiers in the belief that it "settles" the rounds in the magazine and prevents jams.

What I do know is that my old instructors in the British TA specifically told us not to do this, as it might cause the feed clips to deform and that would cause a jam.

Soldiers have been doing that for decades with the belief that it aligns the rounds with the back side of the magazine and therefore decreases the likelihood of a jam caused by a mis-aligned round.

That said, some people point out that this should make no difference to a properly built and maintained weapon.