Memory Gun

The memory erasing gun is a contraption used by the Blind Eye Society to make several residents of Gravity Falls, Oregon forget their encounters with the supernatural.

History
It was created by Old Man McGucket, originally for the purpose of allowing him to forget certain memories, due to the guilt of helping Ford build the Universe portal. He eventually began using it so frequently he lost much of his memory, and eventually became senile. The members of the Blind Eye Society decided to use the device in order to help the citizens of Gravity Falls forget "all the things that go bump in the night."

Season 2
In "Society of the Blind Eye," it is used on Lazy Susan so she would forget about seeing the gnomes. When Mabel got her hands on it she wanted to use it to forget about her past relationships, but ultimately decided not to along with Wendy, who was tempted use it to erase her memory of a rap song. Later, the society members tried to use it on Dipper and his friends when they were discovered, only to fail and get their own memories erased.

In "A Tale of Two Stans" it is revealed that Dipper has kept the gun and gave it to Ford so he can use it to erase the memories of the agency, so that they forget about Stan and the twins before they could discover the universe portal. Ford says "I dunno how you got one of these" which seems to imply that there is more than one. Ford presumably kept it so that he can keep his portal secret.

In "The Last Mabelcorn," the gun is kept in Ford's hidden office. After Dipper discovered fragments of Ford's memories of Bill Cipher he attempted to use the gun in a frantic attempt to eliminate Bill from Ford's mind, but it is revealed that Ford is immune to the guns effects due to a metal plate in his head, so the ray bounces off his forehead and destroys his machine.

Use

 * Memories erased by the gun are stored in "The Hall of the Forgotten."
 * Several members of the society use the gun on themselves, including Bud Gleeful and Blind Ivan.
 * It was used on Ford, but the metal plate in his head stopped any effects of it.
 * List of people known to have had their memories erased:
 * Old Man McGucket
 * Robbie Valentino
 * Lazy Susan
 * Preston Northwest
 * Pacifica Northwest
 * Mrs. Gleeful
 * Jefferey Cannuck
 * Government agents

Side effects
Repeated use of the device results in drastic side effects. The severity of the effects increases with the amount of use, ranging from minor brain damage to total memory loss and eventual insanity. The device's inventor, Old Man McGucket, used it on himself more than anyone else; he slowly went insane and lost all of his memory as a result. McGucket also became immune to any further memory wipes, as seen when Blind Ivan attempted to use it on McGucket. Since the gun is known for affecting people's minds when used very often, the fact that many society members use it on themselves may be the reason behind some of their eccentricities, like the man who married a woodpecker.