This post might got long. I've had to split it.
Have you ever read a fantasy novel with a five or ten page sword fight? Or a superhero novel with a chapter long punch up. These things can be exciting (sure, not when they are a list of blows, but when done right).
Guns are fast weapons. Gunfights tend to be over quite quickly. You can lose a lot of the excitement you are trying to build because the scene ends early. Don't worry, there are ways to keep the story exciting.
Vary the types of gun battles you have. In one scene you might have a character trying to take a sniper shot with a rifle, knowing he'll only get time to fire one shot or the villain will kill someone important to him. In another, your character might be hiding behind cover, fighting several opponents at once. In a third scene, there might be no cover, just one of those Old West style Draws. Each of these scenes have their own way of building tensions, their own style of writing.
2) Get inside your character's head
The best tension comes from being inside your character's head. Let his tensions become the readers. Give them insight into his fears, worries, hopes and dreams. Okay, your MC is the fastest draw in the West. That's boring, though. Where is the excitement when no one can defeat him?
Of course, you don't want him to be defeated, so how can you build the tension? Mentally. Perhaps the character has given up on violence, but is drawn into it again. Now here he stands, in a Draw, about to kill someone. He knows he can, he has before, and he knows he is fast enough... but will he be able to live with himself afterwards? Maybe it would be better just to die.
3) Go omniscient
In 3rd person omniscient POV you can drift from one spectator to the next. What does the whore, the banker, the sheriff, the barman think of the upcoming Draw? That is best done in a Draw situation.