They snatch the wagon in the dark of night a mile from the tunnel. But within that mile of being pursued, the terrain changes dramatically, the sky lightens considerably, and the minimal-length shadows indicate it's no more than an hour before or after noon.
They made the destruction of the rifles and ammo much harder than needed. Since there was a barrel of powder to blow up the wagon and contents, they could have done it at the campsite. Or when they reached the tunnel, they could have done it inside there, rather than by Bill running up the mountain, dodging bullets to blow it that way.