The Rifle was filled with tannerite?
Well I never heard of that, sounds dangerous. Tannerite is generally a target not a propellant.
Excerpt from a much more informative article:
On April 23, 2017, near Green Valley, U.S. Border Patrol Agent Dickey, who was off-duty at the time, ignited a fire that spread to Coronado National Forest, federal land administered by the U.S. Forest Service. This fire was referred to as the “Sawmill Fire.” Dickey started the fire by shooting a rifle at a target. Dickey had placed Tannerite, a highly explosive substance, inside the target, intending for it to detonate when shot by a high-velocity firearm bullet.