Sling Rings: All metal.  There is one on the top of the stock and one on the fore grip.  When slung it surprised me.  Seeing the way the fore grip was attached I didn’t think this would work well but it did and feels surprisingly sturdy to boot.

Other Opinions
I have talked to a few people who have had M60’s and used them for a while, their comments were that the screws that held the Sling rings in can come loose but a bit of thread lock works well on them.