What is the most significant element of Perry and Dick's big "score"? What does their agreement on this element foreshadow?

In the beginning of Part I, the nature of the “score” has not yet been made explicit, but Dick has stolen his father's twelve-gauge shotgun and has fabricated a story that the pair are planning to visit Perry’s sister in Fort Scott. They take to the road, making various stops along the way to purchase supplies: rubber gloves, rope, black stockings. The presence of a shot-gun foreshadows violence or murder.