More than a million high-capacity ammunition magazines flooded into California during a one-week window created when a federal judge temporarily threw out the state's ban, gun owners' groups estimated on Thursday.

Reform groups said the projections are self-serving as gun rights organizations try to make the case that magazines holding more than 10 bullets are so common now that a ban is impractical.

The magazines aren't tracked. But there are plenty of anecdotal evidence that the floodgates briefly opened when US District Judge Roger Benitez overturned the state's nearly 20-year-old ban late last month.

The judge halted sales a week later, but ruled that those who bought the magazines can legally own them while the state appeals his ruling.

'Everything was all sold out. I basically took whatever I could get,' said Chris Puehse, who owns Foothill Ammo in Shingle Springs, east of Sacramento.

More than a million high-capacity ammunition magazines flooded into California during a one-week window created when a federal judge temporarily threw out the state's ban, gun owners' groups estimated (file photo)

He fielded dozens of telephone calls while buyers stacked up 20 deep in his one-man store to buy the hundreds of magazines that arrived in two shipments last week. He had just six left by the time Benitez reinstated the ban last Friday.

'People loved it. It was like we were out of prison and were not treated like bastard stepchildren of the country anymore,' he said.

Thirty-round magazines for military-style rifles, handgun magazines holding 17 to 20 bullets, all selling for less than $30 each - 'they disappeared,' Puehse said. 'They wanted to grab more than I let them, otherwise they would have been gone even faster than a few hours.'

State Attorney General Xavier Becerra warned hours before Benitez again halted sales that California was in danger of becoming 'the wild, wild West for high-capacity magazines.'

'There are those who are now trying to flood the state of California with what were until this decision illegal high-capacity magazines, the type of magazines that are used in firearms to commit the mass shootings that we've seen throughout the country,' Becerra said.

Ruger Firearms announced it was releasing its entire inventory. South Carolina-based Palmetto State Armory announced in a Facebook ad that it was 'prepared to send a whole lot of freedom to our friends in California,' but warned of delays due to high demand.

'The pipeline was open and it was flowing, on all platforms - people showing up (in stores), online - I'm guessing that UPS and FedEx had a field day,' said Gun Owners of California president Sam Paredes. 'It was a frenzy.'

He said an estimate of a million magazines 'seems a little bit low.'

Chuck Michel, an attorney for the National Rifle Association and the California Rifle & Pistol Association, has a photograph of empty racks at a Bass Pro Shops store in Las Vegas that used to hold magazines before Californians flocked to the neighboring state to stock up. California's more than 2.5 million gun owners together have nearly 20 million guns, many of which can use the extended magazines.

District Judge Roger Benitez overturned the state's nearly 20-year-old ban late last month. The judge halted sales a week later, but ruled that those who bought the magazines can legally own them while the state appeals his ruling (file photo)

By week's end, Michel said, 'you couldn't get one anywhere, because all that inventory had been diverted to California. That's probably at least a million magazines, probably approaching two to several hundred thousand people.'

Staff attorneys with the Giffords Law Center to Prevent Gun Violence in San Francisco said they've seen no evidence to back up the million-magazine estimate, and said gun rights organizations have a vested interest in inflating the number of law-abiding owners.

'They have a very specific purpose and intent here to try to set up for the court that these are devices that are very commonly used and possessed' and therefore should not be banned again, said California legislative affairs director Ari Freilich.

'It lets them argue that there's no way to un-ring the bell,' added litigation director Hannah Shearer.

The problem isn't the new owners who will use them legally for target shooting or self-defense, they said: it's that some will fall into the hands of criminals or be used by mass shooters.

The magazines allow shooters to fire more bullets without stopping to reload. Gun owners' organizations - and Judge Benitez - said that's helpful to ward off multiple home invaders, but opponents said it gives victims less time to escape or tackle a mass shooter as he reloads.

Reformers expect the 9th US Circuit Court of Appeals to keep the ban on sales while reinstating a 2016 law and ballot measure banning possession even by those who own them legally.

Opponents are counting on the U.S. Supreme Court to ultimately side with Benitez's ruling that the bans infringe on the Second Amendment right to bear arms.

'Because of that one slip-up, in one week you literally had millions of magazines come into the state that were bought legally,' said Puehse, the gun shop owner. 'These magazines are here to stay.'