I just happened to have a shot out AR15 barrel and a shot out MR556 barrel side by side in a drawer. When I opened the drawer I saw them and thought I should share some pictures as long as I had the chance.
HK uses a longer upper with a longer barrel nut, so they have a longer barrel extension. You can see a pin in this photo. Several of the AR15 derived guns that have been designed for Over The Beach (OTB) work have added a pin to support the extractor while the bolt is in battery. The HK416 and the SIG MCX do this. I want to say there were more guns that did this, but I can’t recall for sure and am not finding any more information online. While the commercial MCX and MR556 have these pins, I do not believe that the manufactures rate those guns for OTB use.
The MR556 has several changes to prevent it from being used in full auto. One of these is a tab on the barrel extension that prevents the use of their barrel with a HK416 full auto bolt carrier. I’ve heard of people throwing these barrels in a lathe to turn off that nub.
I often make fun of German engineering as I feel that they over complicate things. This change I don’t think is a bad one, but it does limit you to only using barrels patterned after their system. Fortunately there are vendors offering aftermarket barrels for the HK rifles for a long time now.