One of the questions I often hear from the gun control side of the argument is "why does it appear that police have an easier time with busting drug dealers than they do with black market arms dealers?" If you think about it a bit more, that's not hard to answer/explain.

When police do a drug bust (a bust in which they use an informant), their informant will usually be safe from identification by the drug dealer because packets of drugs or pills etc look pretty much all the same, and being that in a day's time the average drug dealer could literally have dozens of customers would make it all the more difficult for the dealer to identify who had ratted them out. With black market firearms dealers on the other hand, which in most cases there wouldn't be as many customers day to day, it would be much easier for the gun dealer to remember what type of gun he sold at what time and to who, because unlike drugs the guns do look different.

So that's why black market gun busts are more difficult, because the police do not want to put their informants in jeopardy of having revenge taken against them for ratting out the arms dealers, and it's much easier to keep track of the sale (and what you sold who to) of 4-10 guns that of which at least some are different calibers compared to dozens and dozens of pills or drug packets.