The ornamental design for an instrumentbarrel, 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 rear elevational view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The broken lines showing the writing tip housing in FIGS. 1-5 and 7 and the features on the push button in FIGS. 1, 2, 4, and 5 are included for the purpose of illustrating portions of the instrument barrel that form no part of the claimed design.