I bet someone tried making it full auto by dicking around with the trigger sear. I've seen this (and no, it doesn't work) on these guns before. Check or have it checked before you start randomly buying parts.
What they do is block the sear from holding the hammer back. The hammer follows the bolt and supposedly fires the cartridge when the bolt closes. Not only does it not work, if the bolt stops before battery it's possible for the cartridge to fire out of battery, splattering your right arm and face all over innocent bystanders.
