12 gauge rubber

 is a type of 12 Gauge ammunition that appeared exclusively in Fallout Tactics.

This is the lowest type of 12 Gauge ammunition, use it only if you are left with nothing else.

Behind the scenes
The 12 Gauge Rubber was created for use in riot guns, and is designed to dissuade Chinese supporters from throwing things and breaking stuff. A hungry Deathclaw, however, is another matter entirely.