An ornamentaldesign for a stock adapter for a rifle, as shown and described.
FIG. 1 is a perspective view of the stock adapter for a rifle.
FIG. 2 is a left plan view of the stock adapter for a rifle.
FIG. 3 is a right plan view of the stock adapter for a rifle.
FIG. 4 is a top plan view of the stock adapter for a rifle.
FIG. 5 is a bottom plan view of the stock adapter for a rifle.
FIG. 6 is a front plan view of the stock adapter for a rifle.
FIG. 7 is a rear plan view of the stock adapter for a rifle; and,
FIG. 8 is a cross-sectional view of the stock adapter for a rifle taken at line 8—8 of FIG. 6.
The broken line showing of portions of the stock adapter shown in FIG. 6 is included for the purpose of illustrating environment and forms no part of the claimed design.