Yes its a clamp for a toolless handguard fixation. But i am not quite certain if its not mostly to avoid the HK patent on that. I think it must be quite a dance around all the patents that same folks have signed for 416 ,MPX, Car816,MP 556 etc

The patent density on Ar15  derivatives is getting ridiculous. 

But from what we know now the MK556 was supposedly tested slightly better in tests than HK but more importantly was 52mio Eur cheaper, although that 52mio saving is also down due to some creative accounting on how much longer rifles could last because of the replaceable cam thingy.

Heckler patents and Haenel

Great info, thank you.

Is there any indication of the barrel steel / barrel life for the MK556 vs the H&K416?

Barrels being the major part that wears out first, it would seem that would have been a consideration for overall service life cost. And the 416 is known to have a very long barrel life. 

There are other people who make barrels with that same steel though. HK definitely does not have a lock on that particular thing.

Do you know which other companies are using the same 'HK Steel?"

I seem to recall the steel was French? 

I don't have that information offhand but iirc it's an aermet derivative steel and there's people doing high end CHF barrels using it here stateside.

I'll look at my notes

Aubert & Duval is the supplier. I can't share the exact alloy, but it's a ChroMoVan steel.

The typical German military barrel steel alloy is 32 CrMoV 12 10 and indeed I have seen batches from French steel mills.   

Guess I can post this now, then.

Thanks guys, very helpful.

Do either of you know the H&K RC heat treatment?

I seem to recall that the standard US 4150 was RC 32-34, whereas the H&K was RC 42-46?

I'm a pocket knife nerd originally, prior to getting into guns. Thats an area where steel and RC harnesses are both tremendously important to overall edge retention. 

So I imagine that using a harder RC treatment would also factor into overall 'rifling retention.' 

For example, a typical bayonet is 51-53RC vs 59-61 for a modern, high end pocket knife. That doesnt seem like much of a spread, but theres a huge difference in overall edge retention. Even the difference between 57 and 60rc is noticeable on a pocket knife.

That's about right.