"What did you find down the incline?"One sentence. The plot twist lasts for one sentence! The paragraph break between those two sentences honestly does very little for holding up the dramatic suspense.
"An added twist to the mystery," Frank replied. He pushed his fist forward, and Joe could see the brass glinting through the plastic sandwich bag dangling between his fingers. "Tell me what you see."
Joe glanced as the short rifle cartridges in the bag, noticing immediately the curiously crinkled openings, where the explosions had erupted. "Those guys last night were shooting blanks!"
"Definitely all of those shots were blanks," Frank replied, examining the plastic bag of spent cartridges again.
The rest of the chapter is the Hardy Boys and Eric discussing the mystery. Their discussion is four times longer than the investigation, and they quickly deduce that the villains weren't using blanks, because there's a lot of gunshot damage to their van.