The phrase "muzzle flash" refers to the bright burst of light that comes out of the end of a gun barrel when it is fired. It happens because of the rapid explosion of gunpowder inside the gun. Full definition
As you can see from the picture at the top of this post the game is looking slightly better with a nice muzzle flash on the gun. (thesixthaxis.com)
Just imagine, you get muzzle flash in the front of you, the sound of shells pinging on the floor around you, and the mumblings of a zombie creeping up on your right. (thatvideogameblog.com)
In some scenes, the actors actually fired real Sterling guns with visible muzzle flash from the blank rounds. (howtogeek.com)