Marine embassy guards are among the most professional Marines. It is well known in the Marines that they make grade the fastest of any MOS. Off duty behavior is strictly controlled and there is no faster way to get sent home.
However, all Marine embassy guards are under the direct tactical control of the ambassador or charge d’affaires. If they wimp out the Marines cannot open fire unless fired upon and that may be too late.
Which is a reason for the suits back in DC to not like them. If the Muzzies are coming over the wall, the ambassador may figure he doesn't like the idea of being killed, and order them to fire, without regard to the reaction of the suits in DC.