I had encountered the same when I changed a single gang metal box to a two gang box in the garage. I have to admit I still have problems wrapping a 12 gauge ground wire onto the socket grounding post.
By the way, if you have two prong no ground sockets you can use a GFI to protect the things you plug into the socket. The GFI will trigger and save your equipment in case of a big current draw or overvoltage.
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