Action to Start a Novel?

I read a great start of a novel with an F.B.I. hostage rescue team tight in an alley being slaughtered by a .50 cal machine gun. The level of detail was micro and realistic. As soon as one opens the book intense gunfire is cutting down the highly trained F.B.I. cadre. They drop to the hot alley gravel with spent shell casings flying about, blood flowing, hoping to survive. Just one or two make it to withdraw from the unseen shooter at the head of the alley behind a wall that was working his kill zone. With machine gun bullets searching close overhead time slows down. The omniscient narrator describes what is going through the minds of those still living, and of those fading out. I liked the action level yet the rest of the book seemed somewhat anti-climactic simply because of the intensity of the opening few pages. Sartre’s play; ‘The Condemned of Altona’ occurs mostly in a prison cell and is all action, description and dialogue at one pace that might be regarded as action. Maybe it is more balanced..