Looking at it some more, I think the 1st and 3rd pics are M240's, which is the replacement for the M60.
In particular, look at the end of the buttstock here:
http://www.fas.org/man/dod-101/sys/land/m240g.htm
versus in this thread. Note you can click on the pics at fas.org for higher resolution pics.
Also, note the front sight differences. Pics 1 and 3 in your string have a blade type front sight, while 2 and 4 have ghost ring front sights.