The ornamental design for a sling for a firearm, as shown and described.
FIG. 1 is a top, front, left perspective view of a sling for a firearm showing our new design;
FIG. 2 is a bottom, front, left perspective view of the sling for a firearm of FIG. 1;
FIG. 3 is a top plan view of the sling for a firearm of FIG. 1;
FIG. 4 is a bottom plan view of the sling for a firearm of FIG. 1;
FIG. 5 is a front elevation view of the sling for a firearm of FIG. 1, the rear elevation view being a mirror image thereof;
FIG. 6 is a left side elevation view of the sling for a firearm of FIG. 1; and,
FIG. 7 is a right side elevation view of the sling for a firearm of FIG. 1.
The broken lines showing portions of the design in all figures are included for the purpose of illustrating environment and unclaimed features and forms no part of the claimed design.