I claim the ornamental design for a toy gun barrel, as shown and described.
FIG. 1 is a perspective view of a toy gun barrel showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a top plan view thereof;
FIG. 4 is a left side elevational view thereof;
FIG. 5 is a rear elevational view thereof;
FIG. 6 is a right side view thereof; and,
FIG. 7 is a bottom plan view thereof.
The toy gun barrel is shown with a break to indicate non-specified length. The appearance of any portion of the article between the break lines forms no part of the claimed design.