Ok this is not real criticism just fun nit picking for cynical humor. Don't get mad bro.
We see a marine near the top firing his bolter, with casings ejecting from the right side. We see many other muzzle flashes but no casings elsewhere. We are forced to assume these other brothers have lesser pattern bolters and are getting constantly whacked in the chest by their own empties. Also, that first marine? Two fisting. And the other gun ejects inwards. This is a straight up preference for eating brass.