Try this buffer system

Says for 9mm, but I am sure it can be adapted for 10mm by the correct selection of a recoil spring and the
internal spring. Contact Tim at [email protected], he is very helpful/user-friendly.

1. Familiarity
2. Easy to work on - not many people have the ability to swap barrels on the HK at their house.
3. Parts more expensive on the HK platform
4. Optics options suck on the HK platform vs the AR.  Until recently, there were no good co-witnessing optic options for the MP5.  The new Battle Steel mount from Botach resolves this and I have one and it is great.  However, many manufacturers of new production HK clones are doing a value add by welding the rail to the receivers which ironically won't co-witness with anything so kinda defeats the purpose (of course this is all moot if you don't care about co-witnessing).
5. People that have a registered AR SBR, RDIAS or RR don't want to deal with the hassle and expense to get an SBR MP5 variant or drop $30K for a transferrable FA HK sear or RR.
6. Versatility - for smoothness, I still give the edge to the MP5 over any other closed bolt 9mm SMG.  In addition it suppresses very well...better than my full auto post sample Sig MPX.  However, with the MP5's delayed roller blow back or the gas piston of the Sig MPX, those systems are basically fixed and is what it is.  With advances in various buffers things can change.  I had originally mentioned using the Blitzkrieg hydraulic buffer and you replied:
'As I've yet to see any AR specific hydraulic buffer stand the test of time, so I'll not be a tester. If they were rebuildable, it would be a consideration.'
But then later changed your mind after posting the link to the article from Chris Bartocci using a hyrdaulic as well.

My point is that as things change, buffer/spring designs will continue to develop for the AR platform while the MP5 most likely will not evolve.
In addition, as you see in this thread someone already built an MP5 upper for the AR so it can be done.  
Someone also made a Calico upper for the AR but he did it just to use those helical drums....The Calico uses the delayed roller blow back mechanism similar to the MP5.

Having a delayed roller blowback mechanism + the ability to tune the buffer and springs on the AR would be the ultimate.........
But for now, I'll be satisfied with the 9mm CMMG Guard's radiallly delayed blowback.  Which again is another perfect example of ingenuity occurring for the AR platform.  The Guard's radially delayed blowback also suppresses just as well as my MP5.

The gap is closing on the MP5...

Also, noticed that FM products has a new operating system coming out for the 9mm AR platform: https://www.ar15.com/forums/ar-15/Foxtrot-Mike-s-9mm-upper-anyone-heard-of-them-/15-715196/&page=7
Sounds like some kind of delayed action in the bolt and carrier using the same lower and barrel?

More options the better.