In Dunkirk when Robbie is walking by the man killing off the horses and he turns to walk on the other side of the beached ship, you can see the brown horse start to get back up. 1 hour 6 minutes and I think about 18-20 seconds.

In the commentary they said the reason for this is because they are trained circus horses and they will not stay down for long. The gun is not actually making a noise when the scene was shot.