Derek Chauran (right) looks over a rifle at a Seattle Police Department firearms buyback in Seattle, Washington, Jan. 26, 2013. Seattle adopted a law Monday levying a $25 tax on firearms purchases and 5 cents for ammunition. Reuters

The Seattle City Council unanimously voted in favor of a new tax on the sale of firearms and ammunition Monday night as gun rights activists pledged to challenge the new law in court. Under the measure, gun owners must report any lost or stolen firearms.

The tax will consist of $25 for every gun sold in the city and 5 cents per round for ammunition, which will be used for gun safety research and gun violence prevention programs, according to the Associated Press. The Council adopted both gun safety measures in an 8-0 vote.

"City government can and must pursue innovative gun safety measures that save lives and save money," Seattle City Council President Tim Burgess said, reported the AP. "As it has in other areas of policy, Seattle can lead the way in local solutions."

Seattle's law passed a day after a law mandating universal gun sale background checks went into effect in neighboring Oregon. Senate Bill 941 became law Sunday, expanding background checks beyond only federally licensed dealers to include private person-to-person and online gun sales.

"Background checks are good for public safety," said Eugene Mayor Kitty Piercy at a press conference Monday, reported the Register-Guard. "Closing this loophole makes it harder for criminals to get guns."

The legislative measures were put in place after mass shootings this year in Tennessee, South Carolina and Louisiana and are aimed at reducing gun violence. Gun rights activists, however, plan to legally challenge the laws and stop enforcement.

"The courts aren't going to buy it," Alan Gottlieb, co-founder of the Second Amendment Foundation, a gun rights group, told the AP about Seattle's law taxing guns and ammunition. "This is not authorized by state law, and therefore it's not going to hold up."

Local governments in Washington state are prohibited from adopting gun laws unless they are specifically authorized by state law. Although City Attorney Pete Holmes said Monday that the Seattle measure falls under the city's taxing authority, Gottlieb told the AP that a rule outlawing guns in parks was dropped in 2010 after a legal challenge.

In Oregon, opponents of the background checks have suggested that Senate Bill 941 is unenforceable as there is no centralized registry of guns in Oregon allowing offenders to claim that they bought or sold a gun before the law came into effect. Sheriffs in rural counties in Oregon have said that they will not enforce the law mandating background checks as it will require a tedious investigation for what would ultimately be minor criminal charges, according to the Register-Guard.

An ad campaign paid for by Everytown for Gun Safety supporting the universal background checks in Oregon began airing in Eugene and Portland Monday to combat opposition to the new law.

Seattle gun shop owners have suggested that the tax will not raise revenue but instead push consumers to acquire weapons outside the city.

"The only real purpose of this legislation is to run gun stores out of Seattle," Sergey Solyanik, owner of a Seattle gun shop, told the AP.

There were an average of 131 firearms-related deaths a year in King County, which includes the city of Seattle, between 2006 and 2010, according to Public Health-Seattle and King County.