#60 o ring 
that is the size
don't know the McMaster Carr part #
http://ar15.com/forums/topic.html?b=3&f=66&t=255253
http://ar15.com/forums/topic.html?b=6&f=20&t=227504
that is the size
don't know the McMaster Carr part #
http://ar15.com/forums/topic.html?b=3&f=66&t=255253
http://ar15.com/forums/topic.html?b=6&f=20&t=227504