Fishing for what exactly? Its part of a toy gun and even has your standard "Hey this is a toy gun but dont treat it like a toy!" red sticker on it haha. This screams bad troll more than anything else at this point.
It’s a lower receiver for a replica M4/M16 AEG (Automatic Electric Gun). The boss for the buffer tube is the giveaway that it is for an electrically cycled airsoft gun. There are also GBB (Gas Blow Back) M4/M16 replica airsoft guns where the receiver is harder to distinguish from a real M4/MM16 style rifle, but the dimensions for both the AEG and GBB replicas are purposely different enough so that critical parts are not interchangeable with the real deal.