Democratic legislators have tried to restrict magazine capacity a number of times, most recently in response to the December 2012 massacre at Sandy Hook Elementary School in Connecticut. In that incident, a gunman killed 26 schoolchildren and educators.
In a statement, Assembly Majority Leader Louis D. Greenwald (D., Camden) accused Christie, a Republican who is weighing a presidential bid, of pandering to his party's conservative base.
"At the end of the day, this was a cowardly decision that lacks leadership," said Greenwald, one of the bill's sponsors. "In fact, this is political expediency at its worst, considering the governor is headed out to campaign in Iowa in just a few weeks."
The veto came shortly after family members of those slain at Sandy Hook visited the Statehouse to deliver petitions to Christie in support of the bill.
They had lobbied lawmakers in Trenton both in 2013 and this legislative session, arguing that restriction could help minimize casualties in mass shootings by forcing assailants to reload more frequently.
Christie rejected that logic. "It simply defies common sense to believe that imposing a new and entirely arbitrary number of bullets that can be lawfully loaded into a firearm will somehow eradicate, or even reduce, future instances of mass violence," he wrote.
Gun-rights activists objected during hearings that the bill would violate their Second Amendment right to bear arms and wrongly punish law-abiding citizens while doing nothing to deter violent criminals.
The bill, which passed the Legislature along party lines in May, would have prohibited semiautomatic rifles with fixed magazines exceeding 10 rounds. It would have exempted low-caliber beginner guns. Gun owners would have had 180 days to transfer, surrender, or render inoperable any guns or magazines made illegal by the bill.
Christie noted that New Jersey's gun-control laws are already among the toughest in the country. Last year, the governor approved some anti-gun-violence legislation, such as increasing penalties for gun trafficking, but vetoed others, including a ban on .50-caliber weapons.
As part of his veto Wednesday, Christie proposed a new standard for the involuntary commitment of the mentally ill to include those at risk of becoming dangerous.
He also recommended revising factors to be considered for involuntary outpatient treatment, so that mental-health screeners review whether the person has a history of complying with treatment or has a violent past.
Under Christie's proposal, individuals who have previously been involuntarily committed would be required to produce medical evidence to a court in order to obtain a firearm identification card or permit.
Christie, who proposed similar ideas last year, accused Democrats of being unwilling to engage in a "controversial, challenging, and often uncomfortable public dialogue" associated with mental-health problems.
Greenwald agreed on the need to address mental-health services but questioned Christie's commitment to doing so, saying the administration had "dragged its feet for five years in implementing our involuntary outpatient commitment law."
That law, enacted in 2009, allows courts to mandate people to outpatient treatment if a qualified medical-health professional determines they are a threat to themselves or others.
Democrats say it requires more funding.