In order to get the Magpul MOE or any other M4 grip to work on the HK rifle, you will need to do some moderate amount of Dremeling.
The problem is that the HK grip is not standard and the screw holes do not line up. In addition, the HK trigger has a pin/spring that pushes through the grip as well when the trigger is pulled. If this is blocked, the safety will not work properly nor will you be able to easily pull back on the charging handle.
The tool best suited for this task is a Dremel. I used this bit for the job and it worked like a charm. About $5.00 at Home Depot. If you have a Dremel that came with a set of bits, it should be in your kit. 
In short, you will need to bore out both holes in the stock MOE grip.First take a look at this picture. You will see that the MOE is also "shorter" where the grip need to meet the lower receiver of the rifle. You will need to scrape out with the Dremel (or other tool) about 3/8" back towards the back strap of the grip. This will allow the MOE to fit properly with the lower receiver without having a larger than normal "gap".
The next step is boring out both holes in the MOE so they look like "key holes". You will need to do about 1/4 inch in the key hole. Most likely, you will need to do this several times until you have the alignment right for the lower receiver. When you put the MOE on without screws, perform a function check on the safety/fire selector as well as charging the weapon using the charging handle. If you do not have it lined up right where the trigger/spring pops through the 2nd hole, you will see and feel the grip push down and out of the way.
GO SLOW, this process takes about 30 to 40 minutes to get it just right. Otherwise, you'll end up with a screwed up grip and or a finished product you aren't proud of. When done, it should look something like this. To get the MOE to fit tighter to the lower receiver I also "lowered" the area at the top of the MOE where it meets the lower receiver about 1/64". Use your eyeball to figure this out for yourself and you'll get a really tight fit.Now, you'll notice that by doing this, the head of the factory Allen head screw doesn't have a lot of area to grab onto in the new MOE grip which now fits perfectly. So, what do you do? Build a custom washer of course.
While at Home Depot you pickup a piece of aluminum and you cut it to fit within the MOE grip. Drop it into the MOE and mark where the screw should go. Then drill it out using a 3/8" bit and round it out a little. It should look very much like this.
Almost done. Now take your custom washer and bend the area behind the screw (in my picture to the right of the screw) and put about 45 degrees on this. Why? You will need to allow that trigger/spring to push through and not hit the new shim washer you just made. Otherwise the safety/fire selector will not work. It will also cause the charging handle to feel funny.
Test fit everything while you go. Now put it all together and if you were patient, it should look like this. Now, I mentioned a slight gap and there will be one in the area of the MOE grip closest to the "Safe" word stamped on the lower receiver. Click on the picture and it will enlarge. You can see a small gap there. There's no way to get rid of this gap since the curve in the lower receiver is not "mil-spec" and slightly different than an M4's curve.
I painted the lower receiver with flat black ceramic engine paint and then installed the grip. I also used a small amount of Blue Locktite to keep the screw from even thinking of backing out due to vibration. Now, you have a MOE grip on your UMAREX HK416. Enjoy!
