I dumped some bench notes in another thread...let me check. Jugglez asked me last night and I haven't replied to him yet.
***EDIT***
HERE it is...
Bad news is...I think that they may even be bigger than 8mm...not sure, I didn't mic them.
Good news...they're at least bronze...they'll last longer than plastic, but not as long as sintered/SS metal.
ALSO...these were the G&G M4's from Mach1...I have no idea if there are lower/higher/variations of different models with different build parts...