The ornamental design for a frontsight for a firearm, as shown and described.
FIG. 1 is a partial perspective view of a firearm barrel showing a front sight with our new design;
FIG. 2 is an enlarged detail view of the front sight taken from area 2-2 of FIG. 1;
FIG. 3 is a partial perspective view of alternate embodiment of a front sight with our new design; and,
FIG. 4 is an enlarged detail view of the front sight taken from area 3-3 of FIG. 3.
The dashed lines within the figures are for illustrative purposes only and form no part of the claimed design. The rectangular area formed by the lines on the front site have been shaded to indicate its flatness and contrast relative to the front site.