Despite the title and one mention of Cimarron Pass, there never is an ambush there, or anywhere else.
During the first scene, a lone rider stops his horse, turns, and raises his rifle in the air twice to signal other riders. However, they all were behind a boulder and would not have been able to see the signal.
A soldier is shot in the back with an arrow, then turns around and is shot in the front with another arrow. As he turns, the guide wire for the second arrow is visible against his dark shirt.
(at around 1h 9 mins) When a takedown is performed, a shadow of a camera and camera operator can be seen on lower left corner.
When the rifles became too burdensome, they could have removed a few pieces rendering them useless to the Apaches until the cavalry could return and reassemble them. Veteran soldiers should have known that.