According to Wikipedia and Stephen Hawking's A Brief History of Time, micro-black holes are expected to evaporate quickly due to Hawking radiation, releasing a burst of final emission equivalent to the energy of millions of hydrogen bombs (over several trillion kilotons of energy).
An advanced civilization is in war with another advanced civilization, and is hoping to destroy the latter. The offending civilization (let's call it A) is developing a black hole gun, capable of launching 500-metric-ton black holes. However, they do not have the technology to prevent their black holes from evaporating. Therefore, the projectile must be formed in-situ and immediately fired (kind of like "there are no black hole refrigerators").
This gun can fire, at maximum load and power, black holes at 0.99c. Civilization A has worked out the times for a black hole to evaporate given the time and time dilation. They plan on launching a black hole at their enemy planet in four Earth days. Anything longer and their enemy will annihilate their space fleet. They still need clarification on whether or not to continue the project or ditch it. They ask: "Would this be a viable plan or would if blow up in our faces?"