TALLAHASSEE — A panel of state economists on Tuesday estimated it would cost $4 million to build a registry to carry out a proposed constitutional amendment that targets possession of assault weapons, if Floridians approve the measure in November 2020.

The ballot proposal, backed by the political committee Ban Assault Weapons NOW, would prohibit possession of assault weapons but would provide an exception for people who own the guns at the time the measure takes effect. Those people would be able to keep assault weapons if they register the guns with the state.

Before people could register the weapons, a system would need to be developed under the Florida Department of Law Enforcement. One agency official warned economists on Tuesday that a “number of caveats” could impact the cost of the registry and the time it takes to build it.

“When you are talking about a gun registry, you are talking about potentially millions of guns,” said Ron Draa, the law-enforcement agency’s director of external affairs.

Draa told the panel of economists that they should consider the cost of background checks, about 2,500 work hours for staff to build the system and about $3 million every year to maintain the registry.

While the proposed ballot measure does not require the state agency to conduct background checks on the people registering their guns, Draa said not doing so could be “problematic.”

“If the gun is going to be registered with us, it is probably a liability for us to have a registry with people who should not be possessing a firearm,” Draa said. “From our perspective, we thought that might be a public safety issue.”

If the ballot measure is approved, the state agency would also be required to have the registry up and running within 30 days of passage. Draa told the panel that would be “impossible.”

“Depending on what the Legislature determines a registration system would look like, it could take up to a year and a half,” Draa added.

Panel member Amy Baker, who heads the Legislature’s Office of Economic and Demographic Research, said another “unusual dimension” to the cost of the registry would be accounting for the number of out-of-state people who would move to Florida and register guns.

The registry issues were raised during a meeting in which the economists worked to nail down specific costs of the proposed ballot measure.

The measure defines an assault weapon as “semi-automatic rifles and shotguns capable of holding more than 10 rounds of ammunition at once, either in fixed or detachable magazine, or any other ammunition-feeding device.”

The panel covered the financial impact the amendment would have on government contracts, tourism and sale taxes. For the most part, the panel was uncertain on the exact impact on such issues.

Banning assault weapons has long been a controversial concept in Florida. But a deadly shooting rampage on Feb. 14, 2018 at Marjory Stoneman Douglas High School in Broward County brought the possibility of imposing such a ban into the spotlight again.

The effort gained traction after the Republican-dominated Legislature passed a law last year with gun control measures, but not a ban on assault-style guns. The ban was advocated by Democrats and students from the Parkland school.

For the proposal to make it onto the 2020 ballot, Ban Assault Weapons NOW needs to clear two major requirements. The Florida Supreme Court needs to sign off on the wording that Floridians would see when they vote on the measure, and the political committee would need to submit at least 766,200 valid petition signatures.

As of Tuesday, the state had received 105,062 valid petition signatures, according to the Florida Division of Elections website.

Economists are scheduled to meet again Thursday to continue crunching numbers before signing off on a final tally of the ballot measure’s financial impact.