Iron River by T. Jefferson Parker

In bestseller Parker's disappointing third Charlie Hood novel (after The Renegades), Hood, a Los Angeles sheriff's deputy, joins Operation Blowdown, an attempt to staunch the near constant flow of money and guns across the U.S.-Mexican border. When a shootout during a botched weapons buy leaves the son of the head of a powerful Mexican cartel dead, the fight becomes personal as cartel soldiers cross the border to take revenge on Hood's team. Meanwhile, a faulty product has driven Pace Arms, a family-owned gun manufacturer, nearly to bankruptcy. Unbeknownst to Hood, the man brokering an illegal deal between Pace and another Mexican cartel chief for the production of a revolutionary handgun is Bradley Smith (aka Bradley Jones), the son of bank robber Allison Murrietta, the antiheroine of L.A. Outlaws, the first and best entry in the series.