The ornamental design for a straight-sided barrel, as shown and described.
FIG. 1 is a perspective view of a straight-sided barrel, showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a top plan view thereof;
FIG. 5 is a bottom plan view thereof;
FIG. 6 is a right elevational view thereof; and,
FIG. 7 is a left elevational view thereof.
The dash-dash lines shown in the figures are for the purpose of illustrating environment and form no part of the claimed design. The dash-dot lines in the figures are for the purpose of illustrating boundaries of the design and form no part of the claimed design.