MR556 A1-SD utilizes OSS's suppressor technology which sends back the gas to the first chamber using design that allows reverse turbine-ish performance. The muzzle has a part that works like a compensator which sends gas in to the first part. If the muzzle brake/compensator part is worn out, it can be replaced for a really small fee. The second chamber(near the muzzle) is the serialized part which works as the suppressor.
The shape of the cover is octagonal which dissipates heat so that it doesn't affect shooter's view after prolonged shooting.
Currently they are available for police and military.
HK556 A5 incorporates changes made along the way including adjustable gas block and magwell profile that works with P-Mag. This was carried over to HK762 A2
When asked about interesting keymod design, there weren't a good answer given.
There has been rumors of P30sk and there weren't any. But the rumor is that they may make their appearance in a few months at the NRA show.
