Half of a broken cap to hold a driveshaft in place is my new guess after looking at pics on Google. Pretty clear at least to me being a retired mechanic. A bolt went through the hole. The bolt got loose & the hole got egg shaped and the part broke. You can even see the clearance radius to the right of the hole to allow room to put a socket on the head of the bolt. In any case it's not a gun part. It's a broken chunk of something off a car, truck, farm equipment, etc.