An ornamentaldesign for a stock adapter for a rifle, as shown and described.
The adapter is a specialized connecting piece to attach a stock to a given rifle.
FIG. 1 is a right perspective view of the stock adapter.
FIG. 2 is a top plan view of the stock adapter.
FIG. 3 is a rear plan view of the stock adapter.
FIG. 4 is a right plan view of the stock adapter.
FIG. 5 is a left perspective view of the stock adapter.
FIG. 6 is a bottom plan view of the stock adapter.
FIG. 7 is a front plan view of the stock adapter; and,
FIG. 8 is a left plan view of the stock adapter.
The broken line showing of interior structure in FIGS. 3 and 7 are included for the purpose of illustrating environment and forms no part of the claimed design.