I claim the ornamental design for the offset rail mount for a firearm, as shown and described.
FIG. 1 is a perspective view of an offset rail mount mounted to a firearm;
FIG. 2 is a first perspective view of an offset rail mount for a firearm;
FIG. 3 is a second perspective view of an offset rail mount for a firearm;
FIG. 4 is a top view of an offset rail mount for a firearm;
FIG. 5 is a first side view of an offset rail mount for a firearm;
FIG. 6 is a second side view of an offset rail mount for a firearm;
FIG. 7 is a first end view of an offset rail mount for a firearm;
FIG. 8 is a second end view of an offset rail mount for a firearm; and,
FIG. 9 is a bottom view of an offset rail mount for a firearm.
The broken line showing portions of the offset rail mount for a firearm is included for the purpose of illustrating environment and forms no part of the claimed design.