Originally Posted by Wikipedia

Tannerite is an explosive which is used primarily as a target for firearms. Tannerite is unique in that it is exceptionally stable when subjected to other, less severe (say, a hammer, or dropping) forces. It is generally supplied as two powders which are combined to produce the "live" explosive.

The manufacturer recommends using half-pound preparations of the substance, which should detonate when shot by a high-powered rifle. Small caliber rimfire or slow moving pistol ammunition will not initiate a tannerite detonation.

When Tannerite detonates, it does so at a very high velocity, producing a large explosion and cloud of smoke. It may be useful for persons who are firing at long ranges, in practice situations. A long-range rifle shooter may leave out a number of targets, retreat to his or her firing position, and lay down fire. The shooter doesn't wish to have to walk the possibly several hundred yards down range to see if he has a "hit" -- the Tannerite will detonate instantly, loudly, and serve as an indicator.

Tannerite is often used by Shooting clubs to provide spectacular explosions for their participants in large-scale weaponry demonstrations or other events. Ordinarily, firing high powered machine guns and other rifles won't produce much more than muzzle blast and some sound upon impact of the round on the target. With improvised explosive targets, these shooting clubs can provide a movie-like experience -- exploding cars among other things.

The manufacturer also sells pre-sized and shaped explosives for avalanche control and police uses.