Joe points out that black markets tend to get created by high taxation.
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I discussed this a bit here Firearms Tax and here Gun Control from the Economics Perspective

I don't think that 15% is lower limit of taxation to create a black market. Consider that firearms sold on the black market are probably the result of theft - and thus have no manufacturing costs. The supplier might trade firearms for other prohibited products (like drugs) that have a relatively low cost to produce. It is possible that illegal firearms could actually cost less than the wholesale price of a legally purchased firearm.

A black marketer who sells both drugs and firearms has some big advantages. The costs and complications associated with running an illegal business do not grow appreciably for adding an additional product line.

Much of the drug trade operates with major suppliers or producers at the top of a pyramid like structure, while the illegal firearms trade does not have to follow this model. A 'customer' could simply trade/or sell the dealer a stolen gun. There doesn't have to be a kingpin who bulk purchases firearms and portions them out to his subordinates to sell.

When the police raid a drug house and find firearms it would be difficult to prove that those firearms are for illegal sale, and not for the protection of the drug business.