This isn’t a terribly common occurrence but it does happen from time to time. All of a sudden for no apparent reason you can’t pull the trigger or move the safety on your AR15. The hammer is stuck in the cocked position and will not move no matter how hard you try to pull the trigger.
Nine times out of ten this is because a primer blew out of a case as it was fired and fell into the lower. The primer rolls under the trigger and jams underneath the back of the trigger. If this happens clear the rifle and remove the lower from the upper receiver by pushing out the takedown and pivot pins.
First try shaking the lower upside down while trying to move the hammer, trigger and safety. Sometimes you can jar the blown primer loose and shake it out of the lower. If this doesn’t work than it is a fairly simple matter of removing the hammer and the trigger and removing the blown primer.
First look into the lower and note how the parts fit into the lower receiver to make it easier when you reassemble. Note how the legs of the hammer spring are on top of the trigger pin. To remove the hammer and trigger pins use a punch that is about 1/8″. Punch the hammer pin out of the lower. The hammer will probably be stuck to the rear still. Keep in mind that the hammer is under tension and is going to come flying out of the lower when it is released. So have the lower pointed in a safe direction and also where the hammer can’t get lost when it comes out of the lower. You may have to pull the trigger to get the hammer to release from the lower.
Once the hammer is out you may see the primer inside. You may be able to get it unstuck without removing the trigger at this point by using a small pick or punch.
If not just drive out the trigger pin and remove the trigger. The disconnector will also come out with the trigger. Note its orientation.
You will see the primer in the lower at this point. Also make sure there isn’t any other debris in the lower that might interfere with the proper operation of the rifle.
Reassemble the trigger and disconnector and push them back into the lower. Line up the hole in the disconnector with the holes in the hammer and push the trigger pin back in. Then reinstall the hammer making sure the legs of the spring rest on top of the trigger pin and push the hammer pin back in.
Blown primers do happen from time to time with reloads or factory ammunition so when you find a blown primer it doesn’t mean there is anything wrong with your rifle. However, if you are experiencing blown primers commonly with factory ammunition this does indicate that there may be a chamber problem with the rifle.