The Secret of the Old Rifle — Stephen A. Bly

When the Lewis and Clark Squad discover a trunk full of old rifles--and one with a mysterious code written inside--they're off on their next adventure. What does the code mean? And who could have put it there? The Squad wants to know. Between basketball practices and games they follow up on all the clues. And those clues take them back more than fifty years to a robbery, an explosion, and the deeds of a mysterious stranger. There seems to be a link. But the kids aren't sure what it is. Then, a breakthrough. A girl with family connections to the rifle comes to town. If the Lewis and Clark Squad can catch up with her, they might be able to crack the code and sold this mystery. But then again, maybe the answer lies somewhere else....