I claim the ornamental design for a semi-automatic rifle, as shown and described.
FIG. 1 is an isometric view of a semi-automatic rifle;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a right side elevational view thereof;
FIG. 5 is a left side elevational view thereof;
FIG. 6 is a bottom plan view thereof; and,
FIG. 7 is a top plan view thereof.
The broken lines showing portions of the semi-automatic rifle are included for the purpose of illustrating portions of the article and form no part of the claimed design.