Abstract: A gun lock-out safety device comprises a generally clamshell enclosure and a cylinder key lock. The clamshell enclosure includes two half-shells formed from durable, impact-resistant material, and are sized and shaped to closely fit the contours of the gun to be secured. The half-shells are hingedly connected to form the clamshell enclosure and have marginally interlocking edges to further resist prying and tampering. When properly used, the gun lock-out safety device covers a major portion of the gun including a trigger, trigger guard, breech and/or hammer, and guards against accidental discharge or unauthorized use of the gun.