It's not currently illegal to carry a firearm into a game under Washington law, but CenturyLink Field, as well as Safeco Field and any other stadium, are allowed to prohibit folks from bringing certain items, including firearms, into said stadiums. House Bill 1015, proposed by three Republican members of the state House of Representatives, would eliminate the stadiums' ability to "prohibit persons with a valid concealed pistol license from carrying a concealed pistol in any facility or on any grounds of a facility." The league doesn't allow most bags in games, requiring fans to utilize clear plastic bags to bring items into games.
– Lawmakers in Washington state are floating a controversial idea to let sports fans bring guns to the stadium. Under the proposed amendment to state code, private entities that operate stadiums would not be able to prevent people with licenses for concealed guns from entering while armed, reports CBS Sports. In this case, the law would cover CenturyLink Field, where the Seattle Seahawks play, and Safeco Field, where the Mariners play. Not on board with the idea? The NFL. “We haven’t seen the proposed legislation but we have a policy forbidding carrying a weapon into NFL stadiums,” the league's vice president of communications tells the Washington Post in an email. As the Guardian notes, "there would be obvious concerns about allow firearms into venues with alcohol and where passions can ignite."