Police confiscated it from a man who said he bought it for $100 from another person just outside the event.
It is illegal for anyone besides military personnel to have a missile launcher.
The launcher was checked and no missile was inside.
Officers ended up trading a $100 gift card for it.
Police said they will return it to Joint Base Lewis-McChord and launch an investigation.
Police said 716 guns were turned in in less than four hours, including 348 pistols and 364 rifles. Among the weapons turned in were three “street sweepers,” shotguns that include a high capacity magazine capable of holding twelve 12-gauge shotgun shells.
At one point, the buyback had to shut down for a short time so officers could get more gift cards.
“I think it was good to get it out of the house. One, there’s no accident that could happen now. Two if there’s a burglary, the gun’s not there,” said Jack Jolly, who turned in a gun at the event.
In comparison, a four-day gun buyback program in 1992 in 1,100 guns.
Seattle Mayor Mike McGinn said at a Monday news conference that officers handed out about $68,000 in gift cards that were donated by businesses.
Rifles, handguns and shotguns were eligible for up to $100 in gift cards, and assault weapons were eligible for up to $200. Several members of the public dropped off unwanted weapons and did not want a gift card in return.
Not all people who were at the event thought it was a good thing.
“It’s a silly idea because these people that turn the guns in are not the ones that are the problem in the country today,” said Robert Hough.