The ornamental design for a fieldhunter's gun prop, as shown and described.
FIG. 1 is a top front perspective view of a field hunter's gun prop 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, the right side being symmetrical thereof;
FIG. 5 is an enlarged top plan view thereof; and,
FIG. 6 is an enlarged bottom plan view thereof.
The broken line showing of a rifle and ground plane depicted in FIG. 1 is included for the purpose of illustrating environment and forms no part of the claimed design.