There is a tradition, right across the Multiverse, of macho law-enforcers toting ridiculously heavy sidearms, which surely represent overkill for normal policing duties. Dirty Harry had his .44 Magnum; Detritus carries a siege weapon with a two thousand pound draw.
As Detritus considers this a waste of energy if all the energy devoted to drawing back and cocking the string is only expended on a single arrow (which from a human point of view is a six-foot lance), he has adapted it still further, so that it shoots a bundle of twenty or so arrows bound together around a central core. Due to the violent force acting on them, the arrows tend to disintegrate into a cloud of supersonic (a significant fraction of local lightspeed) shrapnel which bursts into flame from air friction. The resulting fireball scythes everything in its path totally clean. Hence the name. It not only opens front doors, but frequently creates a back door as well.
Especially as Detritus' first reaction on picking the Piecemaker up from the city armoury was to ask "Which bit am der safety catch?", Samuel Vimes has effectively forbidden him from firing it anywhere within the City. But anyone he points it at, in the normal course of policing, doesn't need to know that. Especially with Detritus' haziness about the nature of safety catches and the tortured noise as of metal under great stress that the loaded weapon emits... well, accidents can happen.
Detritus had his chance to see what it could do when fired in anger during the showdown with the Überwaldean Werewolves in The Fifth Elephant. The first shot not only shatters the doors into pieces, but it takes a substantial part of the castle frontage along with them. The second shot, fired in more haste, creates even more structural damage and leaves Serafine von Überwald with not only the werewolves' plot in ruins, but their castle requiring extensive rebuilding.
Piecemaker Mark IX
The Burleigh & Stronginthearm Piecemaker Mark IX appears in Snuff, as used by Willikins. It appears to be a small, foldable crossbow that makes no noise when shot, and can be easily concealed within a pocket. Only three of these were ever produced, due to being deemed so dangerous, two of which are kept under wizard-assisted lock and key in the Burleigh & Stronginthearm vaults. The third one was kept in a secure vault in the cellar of the Ramkin residence on Scoone Avenue, and it is this one that Willikins seemed to have obtained.
The penalty for being caught carrying one of these in Ankh-Morpork is to be hanged, unless said person is caught by a member of the Assassins' Guild, in which case (the Guild have implied) hanging would seem like a walk in the park. The reason Wilikins has escaped this fate? Well, for one thing, even his already deemed unkillable master, Samuel Vimes, is afraid of him.
The Roundworld reference is to the Colt Peacemaker .45 revolver, a staple weapon of law enforcers and criminals alike in the American Wild West and - similarly to Detritus' weapon - made and kept the peace by making it impossible for the other side to continue being aggressive, which is difficult to do when you're dead.
Other policemen in fiction with unfeasibly large weapons:
- "Dirty" Harry Callahan (Clint Eastwood) in Dirty Harry wields the Smith & Wesson Model 29 .44 Magnum revolver, a weapon somewhat overpowered for police work and better suited for hunting large animals, like five hundred pound grizzly bears.
- Legendary US Marshal Wyatt Earp supposedly (in fictional accounts) carried a "Buntline Special," a Colt .45 Peacemaker with a 12- or 16-inch barrel.
- Macho cop Lieutenant "Sledge" Hammer, toting his trusty "amigo" in a TV series that was effectively Dirty Harry played for laughs. Hammer, when faced with a random rooftop sniper, once decided the Amigo was not nearly enough, and got an anti-tank missile from the boot of his car, succeeding in demolishing the entire seven-storey building. Watch it here: 
- In the TV series Wanted: Dead or Alive, bounty-hunter Josh Randall (Steve McQueen) carries the "Mare's Leg," a Winchester rifle cut down into an oversized pistol.
- In Tom Sharpe's stories about the amoral, corrupt, criminally inefficient and cretinously led South African police force of apartheid days ("Indecent Exposure" and "Riotous Assembly"), the worst constable in Sed Effrrika, Constable Els, who is a sort of Afrikaaner Nobby Nobbs stripped of all redeeming virtue, totes a mighty four-barreled hunting rifle used originally to down a charging rhino, with truly spectacular effect.
- In Bernard Cornwell's Sharpe tales, about a proto-commando in the Napoleonic Wars, the huge and hulking Sergeant Harper totes a fearsome seven-barreled musket, originally designed for close-quarter naval use with the express intention of bringing down a battleship's mainmast and rigging. This failed as a naval weapon because sailors tend to be smaller and wirier men whose talents lie elsewhere. Besides, to fire a weapon like this from the deck of a rolling and pitching ship could be... something of an own goal. But the huge Harper, who has a suspiciously Detritus-to-Vimes relationship with Captain Sharpe, takes to it like a troll to a siege weapon. (Memo: Cornwell may be a Discworld fan?)
- Dennis Sylva of Taylor Anderson's Destroyermen series carries a rifle made from a 25mm (1 inch) anti-aircraft gun; he has affectionately dubbed this the "Doom Whopper."