The ornamental design for a "weapon", as shown and described.
FIG. 1 is a left side view of a weapon constructed in accordance with the teachings of the invention.
FIG. 2 is a right side view of the weapon of FIG. 1; and,
FIG. 3 is a left, front perspective view of another weapon constructed in accordance with the teachings of the invention but employing a different scope than the weapon of FIGS. 1 and 2.