If it cuts nice it is probably Delrin. If it comes off stringy and the chips are hard to break, like a rubber band, its UHMW or something like that.
But I think I would sell it and get a new stock, and put a few bucks in my pocket for some ammo.
Are you sure its Delrin? It should machine pretty easy. Could it be another form of plastic?
If its melting slow down your RPM or surface footage and increase your feed.