Economics of Violence

 refers to an unmarked quest in Fallout 3.

Pronto in Paradise Falls complains about having no good items to sell as all he gets from the other slavers is leftover junk. If you offer to help him out he will ask you to bring him 20 Chinese Assault Rifles.

Chinese Assault Rifle locations

 * On Super Mutant Masters (in Vault 87, for example).
 * Inside Mama Dolce's factory. The factory is full of Chinese Remnant soldiers and virtually all carry this weapon.
 * Talon Company mercenaries often carry these weapons at higher levels or difficulty settings. Fort Bannister has frequently-respawning mercenaries with this weapon.
 * Megaton's Armory houses three Chinese Assault Rifles.
 * Stealing individual weapons from certain characters is underhanded, but will work. Lucas Simms in Megaton carries the weapon, as well as Harkness in Rivet City.
 * Flak 'N Shrapnel's in Rivet City usually will start out with two of the rifles, and a third can be found in their room on the upper deck (Average Lockpick required).
 * In the Point Lookout add-on, there are several in the bunker at the end of The Velvet Curtain quest.
 * There are also a good number in the Capitol Building.
 * There is one on the shelf in the Wilk's house in Grayditch. (from the quest Those!)
 * 4 or 5 can be looted from slavers at Lincoln Memorial, during Head of State.
 * In The Pitt add-on, three can be found on a shelf in the south-west room on the second floor of the Abandoned Apartments.

Rewards
Pronto's shop will have a better inventory and he will also get a bit more cash (around 400 instead of around 150).

You will also receive a "business partner" discount of 20%.

Bugs
Once you give him 20 rifles, if you talk to him about his inventory again and agree to get rifles for him, his inventory could revert to the previous version. So don't talk to him about his inventory once you have given him the rifles.