Radley Balko on race and gun laws
Among other things, he discusses a form of "sting" which has become very popular, and given rise to the concept of "sentencing entrapment." It goes like this: Federal agents recruit an informant and send him out. He tells others that he knows where there is a drug stash house, and he wants to form a gang to rip it off. The house and drugs actually exist only in his imagination. Others agree to join him. He specifies that one or more of them must bring a gun.
Then they charge everyone except the informant with conspiracy -- not to rob, but to possess the drugs for sale (since in any noteworthy quantity, possession for sale carries higher penalties than armed robbery). Since a gun was involved, the charges tack on penalties for that -- 5-10 years consecutive, without probation. The Federal Sentencing Guidelines for drug offenses go up steeply with the quantities involved, and since the drug stash house exists only in the informant's imagination, he is free to invent any amount supposedly to be found that, and thus increase the sentences as far as he desires.
One judge (I think it was Posner) has pointed out that the effect of these stings is to protect drug stash houses, which seems a bit paradoxical.