The ornamental design for a barrel for writinginstrument, as shown and described.
FIG. 1 is a perspective view of my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a right side view thereof; the left side view being a minor image;
FIG. 4 is a rear elevational view thereof;
FIG. 5 is a top view thereof; and,
FIG. 6 is a bottom view thereof.
The horizontal line extending across the barrel of the writing instrument defines the bounds of the claimed design. The outline of a writing instrument shown in dashed lines in FIGS. 1, 2, 3, 4 and 6 illustrates environmental matter. None of the dashed-line subject matter forms a part of the claimed design.