The Patriot Game — George V. Higgins

For Pete Riordan the assignment is both simple and formidable: find the IRA leader who''s running around the US trying to be guns. (Nobody knows his name and nobody knows what he looks like.) There is one possible lead: two Massachusetts politicians are trying to bring off an early pardon for Mikey-mike Magro, currently in prison for murder. And Magro is known to very proficient at finding guns for sale.