We claim the ornamental design for a barrel for a writing instrument, as shown and described.
FIG. 1 is a perspective view of a barrel for a writing instrument showing our new design;
FIG. 2 is a front view of FIG. 1;
FIG. 3 is a back view of FIG. 1;
FIG. 4 is a right side view of FIG. 1, the left side view being a mirror image of the right side view;
FIG. 5 is a top plan view of FIG. 1; and,
FIG. 6 is a bottom plan view of FIG. 1.
Portions of the barrel for a writing instrument shown in broken lines on the top, bottom and three raised features on the grip in FIGS. 1 to 6 are included to show features that form no part of the claimed design.