This is a difficult one. I think what you gotta remember is that the opening will set the tone for the whole book. If your book is going to be action oriented that's what you've gotta start with. Action books are a series of climaxes tied together or provoked by plot. Good action novels tend to start with a small climax that introduces characters and end with a big climax that ties up the ends. I'm gonna point at books by Matthew Reilly, Robert Ludlum and John Birmingham. Someone (often multiple someones) always gets gunned, blown up or eaten in the first few pages.
On the other hand if your book is more plot, character or dialogue driven, then this is what you begin with. I'll point to John Le Carre in this case. He spends the first few pages establishing characters, motives, plot and context. 'The Spy Who Came in From the Cold' and 'Call for the Dead' are prime examples of this.
I'm guessing (given the context you described) that it's gonna be more action based (see above). If it's a mixture of both, I'd suggest reading authors like Cormac Mccarthy, famous for 'the Road' and 'No Country for Old Men', but I'd recommend you'd be better off checking out 'Blood Meridian', a western, but it has some gruesome action (the scalping scenes are poetically detailed), incredible characters and multifactional conflict (though it can be difficult to read). Another good author is Michael Ondaatje, and his books; 'The Enlgish Patient', 'In the Skin of a Lion' (prequel to the former, should be read first, fantastic book) and 'The Collected Works of Jessy James' (I believe that's the title).
