Compared to MK2 upper and handguard are much more streamlined. Overall MK3 is way more aesthetically pleasing, lines, fit and finish. Stock is significantly better, it is still 1913 attachment, but I think its some 3rd party, quick push button removable. Charging handle has a smaller hook I think one piece, not sure, instead of large round barrel knob. New piston, new carbine length rod is one piece, really stubby and short. Bolt carrier is significantly different from usual 180s, seems to be way more elegant compared to previous Mk2, can't say, but it appears that bold carrier lost a lot of weight. Return springs are captured, bolt carrier is getting out with return springs and rods as one assembly. Nevertheless return springs and rods can be removed and installed from/to carrier without tools. Springs back plate appears to be different.
Bolthead and piston are either chrome plated or are in white metal unfinished. Didn't ask. Was thinking about something else.
Oh yeah, 9mm is cute too. Same upper, handguard, stock overall polish as MK3. Feeds from their own metal magazines 5/10 round. Look like tiny mp5 mags. 9mm barrel is not full length rifled to maximize velocity and has end section with ports cut right into it. They are quite long and it looks somewhat strange.