The ornamental design for a boltknob for a bolt-action rifle, as shown and described.
FIG. 1 is a perspective view of a bolt knob for a bolt-action rifle showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a one side elevational view thereof;
FIG. 5 is another side elevational view thereof;
FIG. 6 is a semi-perspective top view thereof;
FIG. 7 is a semi-perspective bottom view thereof; and,
FIG. 8 is a view of the bolt knob positioned on a bolt-action rifle.
The broken line showing of aspects of the claimed design is included for the purpose of illustrating environment and forms no part of the claimed design.