The ornamental design for a back cover of massagegun, as shown and described.
FIG. 1 is a rear, right and top perspective view of a back cover of massage gun, showing my design.
FIG. 2 is a front elevation view thereof.
FIG. 3 is a rear elevation view thereof.
FIG. 4 is a left side elevation view thereof.
FIG. 5 is a right side elevation view thereof.
FIG. 6 is a top plan view thereof.
FIG. 7 is a bottom plan view thereof; and,
FIG. 8 is an enlarged view of detail 8 in FIG. 1.
The evenly-dashed broken lines depict portions of the back cover of massage gun that form no part of the claimed design. The dot-dash broken lines in FIGS. 1 and 8 depict the boundaries of the enlargements that form no part of the claimed design.