SEATTLE (AP) - Seattle's City Council has approved a new tax on the sale of firearms and ammunition - a measure designed to help offset the financial toll of gun violence. But opponents quickly promised a legal challenge. The council adopted the tax on an 8-0 vote Monday, along with a companion measure to require mandatory reporting of lost or stolen firearms. The tax amounts to $25 for each firearm sold in the city, plus 5 cents per round for nearly every type of ammunition. Council President Tim Burgess said the measures were designed to deal with gun violence. But gun-rights activists promised to sue on the grounds that the city doesn't have the authority to adopt the measures. Washington state prohibits local governments from adopting laws related to firearms unless those local ordinances are specifically authorized by state law.