This is repost (approved by one of mods) from my HKPro post. Of course there are no MR223 in US, but MR556A1 is different only in details.
My friend managed to shoot one bullet with other one (squib) while teaching carbine class using my MR223. Rifle run fine after than, only visible damages were broken firing pin retainer and destroyed baffle on muzzle brake. However he declared to exchange my MR for new one. He is man of his word and we located unfired MR223 from second production run in 2009 (my old was from first production run - number 0004xx).
Main differences for production runs after first one are those known from MR556A1:
- Ball detent take down pins.
- Stock with lock screw.
- Shaved sling loops from gas block.
But it still keeps (up to model 2011 that has same features as MR556A1):
- Crappy single stage trigger (not so crappy as first run, but still).
- 416 handguard screw, that can be manipulated by using BGC.
One more difference between them (but not known to MR556A1) is that while both have M15x1 thread for muzzle device, first batch had 45dgr shoulder (good for use on HK G36 flash hider) while second batch have square shoulder that works better with crush washer (AR10 crush washer needed).
Here they are both, with old one on top on way out (Micro T1 stays with me) and new one on bottom of picture.
I also decided to document how internals of not properly maintained and stored in humid environment MR223 looks after some 6000 rounds. One word of advice regarding non-lined HK barrel. After some 6000 rounds it started to keyhole one every shot with surplus 5.56x45 NATO ammo (Santa Barbara from Spain). It was still shooting fine with 77gr HPBT .223rem ammo. Initially I assumed this could be result of squib incident, but it looks like using barrel foam and 3 passes of bore snake as only way of barrel cleaning is just not enough. After using some MPro7 Copper Remover and JB paste barrel shoots back like new. So, note to myself, use copper remover on un-lined MR barrel at least once per 5000 rounds 
Here are some pictures of my MR223 internals after brief cleaning (using paper towels) to show up wear usually masked by dirt.
First lets deal with “carrier tilt”. Some pictures of inside of barrel extension (white lines are actually coating discolouration, that were there day 1), buffer face and bolt carrier skis for signs of excessive (or any) wear caused by carrier tilt:
It is clearly visible that carrier skis do some work, but I would hardly call it wear. Even with DI system carrier drags inside barrel extension. Except of that I can see… nothing.
Now lets look on damages that bolt cam pin does to upper receiver:
Two things here. First is relief that HK made in upper receiver to allow cam pin to move without dragging on upper receiver. Second is actual wear close to egde of this relief. By very scientific method of analyse (running finger over it) I determined that this wear is mostly removed surface coating but small shelf can be felt. Also this wear developed over first 500 rounds and stays this same way since then.
Now have a look how BCG and barrel extension are doing:
Both seem to keep up. You may note two features there, that are part of OTB package: drain hole in bolt carrier and pin machined in barrel extension, that supports extractor. It does not mean MR223 is OTB capable (and I’m not willing to test) - it just has features from OTB capable 416.
I also tried to take picture of barrel bore (actually before it was de-coppered), but I seem not be able to do so with my equipment and (more importantly) lack of skills needed. But here are best that I managed.
I hope that this may came informative for someone.
“what!? do I need to make 12000 passes of brush?”