I claim the ornamental design for an adapter for coupling a grenade launcher to a handguard, as shown and described.
FIG. 1 is an isometric view of an adapter for coupling a grenade launcher to a handguard, showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a left side elevational view thereof;
FIG. 5 is a right side elevational view thereof
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
Broken lines are for illustrating unclaimed portions of the adapter for coupling a grenade launcher to a handguard; the broken lines form no part of the claimed design.