The Mini-Sentry is a replacement for the Sentry Gun in Team Fortress 2. It can only be placed by engineers wielding the Gunslinger, a replacement for the wrench. It requires 100 metal to build rather than the usual 130 metal necessary to build a normal sentry gun. It cannot be repaired or upgraded, only restocked with ammo.
Like all sentries in TF2, the sentry will build itself automatically, though slightly faster than the normal level 1 sentry. This can be sped up by hitting it with the wrench.
It fires at twice the speed of the normal level 1 sentry gun and has slightly more health. This makes a great deal more deadly.