just wondering if anyone here knows if the MR223 will accept 5.56 nato rounds.
Up here we can easily purchase the MR223, but the MR556 is a little tricky.
I’ve tried contacting H&K directly, but the answer i recieved was rather nebulus…
any and all help is greatly appreciated
I had the same question re 5.56 x 45 use in an MR223 last spring. Asked over at hkpro.com. My own hunch:
“It would make sense for ease of production (same barrel specs and tooling as the HK416) and liability (5.56 x 45 chambers can safely shoot both 223 Remington and 5.56 X 45 but not the other way around).”
And the following reply from skylinedrive convinced me to give it a go:
“I don’t know about the SL8, but the MR223 is chambered for the 5.56 Nato. It is written so in at least two german gun magazines, they would not write it if it weren’t so because they would get their arse sued over it.
I shot 800 rounds of 5.56 through my .223 without any problem. It’s hard to say why HK choose to call the european version MR223, maybe to make it sound more “civilian”, everybody over here brings the caliber 5.56 Nato into relation with the military.”
Only you can decide for yourself, but I happily bang away with 5.56 x 45 in mine now.
If you have $44 plus shipping, taxes, duty etc, and want to really know the answer, not just get anonymous intartube advice from guys like me, get one of the following:
ht tp://www.m-guns.com/tools.php
see the “.223/5.56? Gage” on the main page
“This gage tells you if you have a chamber that is 5.56 NATO (or larger), or something shorter/smaller.”
If you do get one, please post your results. Facts beat opinion, even if the opinion is mine 
Check out this thread. https://www.m4carbine.net/showthread.php?t=40807&highlight=5.56+reamers