Inside, Buckley found Bud Dawson utterly ignoring a bullet wound in his shoulder, while he feelingly wept at having to explain why he failed to drop the "blamed masquerooter," who shot him. At the entrance of the ranger Bud turned appealingly to him for confirmation of the devastation he might have dealt. "You know, Buck, I'd 'a' plum got him, first rattle, if I'd thought a minute.
This is from "Afternoon Miracle" by O. Henry.
I can't understand the meaning of this.