I'm assuming you have a Ruger adjustable target rear sight. Yes, you can tap it back on center. Just to the left of the sight's elevation adjustment screw there's a little slotted screw in the base of the sight, this is the hold down screw for your target sight. Remove that little screw and put some Locktite (blue) adhesive on it and then reinsert it into the slot and tighten it. Leave the gun and sight alone for a minimum of 24 hours and you shouldn't have a problem with your sight coming loose again.