The HK accepts any mil-spec parts and on my rifle, I chose to add the following Magpul products.
MAGPUL MBUS REAR FLIP SIGHT - I removed the factory HK sights and replaced them with these flip up backup sights. Very nice, the polymer seems to be very tough and durable and resistant to gun cleaner.
Magpul AFG2 Angled Foregrip - Instead of the standard "stick" vertical grip, I chose the AFG since I seem to hold the rifle typically by the mag well and the AFG fit very naturally with the way I hold the rifle to shoot. Its also quite popular at the range with the AR crowd.
Magpul Milspec Carbine Butt-Stock - This is the upgraded stock from the MOE series with the extra clamp at the front to take out any free play. On my XM177 and M4, the CTR grips like a rock with ZERO play. On the HK, there's still a tiny bit due to the buffer tube being slightly undersized.
Magpul Moe Drop-In Pistol Grip - This grip WILL NOT fit on the HK without modification. You will need to use a Dremel tool to create the proper screw hole alignment and bore out the back of the MOE to fit the HK's lower receiver. The HK also has the hammer spring which pushes down into the grip and you will need to bore this out of the MOE grip in order for it to work properly. If you do not do this, you will find that the safety will not work as well as pulling the charging handle. You have to bore out both holes to make the MOE work. With a Dremel tool, this did not take long at all - 40 minutes. I also had to create a custom washer to ensure that the pressure from the grip screw is distributed along the entire length of the grip. After doing that work, the curvature also left a 1/64 gap between the grip and the lower receiver - this is due to the curve being slightly different than a typical M4/AR15. A little black paint and it looks awesome. I wish I had some black Guncoat to brush over instead of using a ceramic engine block spray paint. Detailed pictures forthcoming.