Knowing a little about these kinds of weapons from the decade I spent in the US Army, I assumed that he was hit by a ricochet and hemorrhaged internally, which is why it took so damn long.
My brother and I were talking about it today, and he expressed a post-finale desire to maybe have seen Walt find his money still on the back of the truck and drove off behind Jesse with it to live happily ever after somewhere. The wound could easily have been superficial because we really didn't see that much of Walt's blood. But, of course, we realize that the implication was that he died, so he died. And even though we never saw Lydia's bones, the implication was that she would die, so she died. Anything they go out of their way to state in a finale like this, such as Elliot and Gretchen's delivery of Walt's money to his son, is what will canonically happen.
And it really did end the right way. I don't think there was a better way for the show to end. Even though Walt died, the audience is left with a happy feeling because of Walt's demeanor and the tone set by the music. It is a happy ending for the audience, which, ultimately, is all that matters.