Missoula police said a man suspected of trying to sexually assault another man pulled what they believed to be a handgun on them early Sunday morning.
The handgun turned out to be a BB gun.
But police said at first glance it looked like a black 1911 .45 pistol.
Police were responding to an attempted sexual assault report on the 2000 block of Cooper Street.
The alleged victim had left the residence.
Police said the suspect came to the door and pointed the gun at officers.
They said he dropped the weapon when he was ordered to, and was taken into custody.
The 20-year-old man faces charges of misdemeanor attempted sexual assault, felony assault with a weapon, and criminal trespass to property.