Iain [M.] Banks - Against A Dark Background

For Culture fans note that while this is SF this in not a Culture novel.

Lady Sharrow, a former pilot and antiquities thief, lives in exile on a planet called Golter, which, with its sun Thrial, lies a million light years from any other star. An eccentric cult, the Huhsz, believes that the messiah can not be born until the end of her bloodline. The only way to escape their hunting passport lies in the recovery of the last Lazy Gun – an ancient weapon which has a surreal and humorous approach to death.

Sharrow, who as a child narrowly survived the assassination of her mother, joins up with her former combat team on a series of quests. The group locate a number of Antiquities – including the Crownstar Addendum, one of the most valuable pieces of jewelry in the solar system, and the Universal Principles, an ancient book – on the way to the final quest for the last Lazy Gun.
Lazy Guns

The eight Lazy Guns were mysterious ancient objects, discovered together a long time before the events of the book.

A Lazy Gun is roughly half a metre long, 30 cm wide and 20 cm tall. The Lazy Gun is "light but massy", and weighs three times as much when turned upside down. The Lazy Gun is the only weapon known to display a sense of humour.

When the Lazy Gun is fired at humans, many different things may occur. An anchor may appear above the person, giant electrodes may appear on either side of the target and electrocute them, or an animal may tear their throat out. Larger targets such as tanks or ships may suffer tidal waves, implosion, explosion, sudden lava flows or just disappear. When fired at cities and other such targets, thermonuclear explosions are the norm, although in one instance a comet crashed into the city.
Story

Seven of the original eight Lazy Guns were destroyed before the events of Against a Dark Background. One disappeared with its user when he tried to fire it at the local sun, one suffered a lucky strike during an air raid, two self-destructed when investigators tried to take them apart and another was destroyed by an assassin. A sixth was destroyed when investigators fired it with its lenses looking through an electron microscope. The seventh, found before the events of the book by the Lady Sharrow and her team, was destroyed by the university it was sold to when they tampered with it. The resulting explosion devastated the city the university was located in.

The hunt for the eighth and final Lazy Gun is the main plot theme of Against a Dark Background. Much of the novel concerns Sharrow's adventures in searching for and acquiring it. Her motivation is that the Huhsz religious cult regard it as a sacred object, and that if she can find it and give it to them, their vendetta against her will lapse.

Sharrow encounters various political systems on her travels across Golter. She also meets the Solipsists, a gang of pirate mercenaries on a hovercraft, who hold very unusual philosophical beliefs.

When the last Lazy Gun is eventually discovered, it is guarded by an elaborate defense system incorporating a genetic key which Sharrow has to deactivate.