Can't comment on the HK416D but I recently got an HK416A5. I had heard the horror stories so I went into it being prepared to overhaul it internally if necessary, but it was actually pretty decent for the most part. That said I did find a couple of things that can't really be excused, although they weren't a bother to me personally as I had expected to swap the affected parts out anyway as part of fine-tuning it.
These were my initial observations:
The look & feel is the best on any AEG I've held, let down only by the "licensed trademark..." and "Cal. 6mm BB" markings, although at least the latter is discreetly positioned (small text inside the trigger guard) - it would be so much better if these were just stickers you could easily peel off though. Exterior finish is excellent and seems quite hard wearing, weight is good and the upper & lower recievers fit together with barely any gap and no wobble - it feels very solid.
The right side selector is a bit loose, which is slightly disappointing but AFAIK a common issue with this kind of ambidextrous selector design.
Stock was very stiff at first but has started to slide more smoothly with use.
While it is recommended to use a LiPo battery, when it arrived I didn't have any LiPo packs that would fit (the battery fits into the buffer tube). I did have an 8.4v NiMH stick battery though and found that it works just fine and will fit as long as the stock is extended a few places. The battery connector is unfortunately a mini-Tamiya type.
~311fps out of the box, no idea about accuracy as I couldn't test that at the time & didn't want to delay my work on the internals for it.
Once I was satisfied that there weren't any obvious issues with it I started with the warranty voiding. First I looked at the barrel/hop setup as that's easiest to get at.
The inner barrel is brass, 285mm long and AFAIK 6.03mm; it was *filthy* but looks decent enough and performed very well when I tested with it after fine-tuning everything.
Hop unit is a plastic rotary style unit & seems decent enough.
The stock hop nub is a hard plastic cylinder.
The stock hop rubber doesn't seem all that good and had a couple of blobs around the feed lips that would compromise the airseal.
While the barrel and hop unit itself are ok, the stock hob rubber and nub were pretty poor TBH. Just swapping the stock hop rubber out for a Maple Leaf Macaron with Omega nub increased the power by about 10fps and probably made it far more accurate as well.
My original plan after doing the barrel assembly was to just throw in a Guarder SP90 spring and see how well it did before doing anything else, as the gearbox has a quick change spring system that's supposed to allow the spring to be changed through the stock tube. Videos and posts about this show that it can be done by removing the stock, unscrewing the buffer tube cap and then releasing the spring guide assembly by pressing in and rotating the back of it 90 degrees CCW using a hex driver.
On mine turning the screw to remove the cap removed the both the cap and spring guide assembly. On further inspection this was because an O-ring had been put over the screw & squashed between the buffer tube cap and the back of the spring guide, and since they're both turned in the same direction to release & you need to keep some pressure applied to turn philips head screws it ended up releasing the spring guide.
Unfortunately I was unable to swap the spring out though as it was attached to the piston, so I had to take out the gearbox anyway.
Inside the gearbox:
As expected, I had to disassemble the piston to remove the spring.
I found a metal shaving on the piston O-ring 😕
Nozzle is an air seal type, looks fine
Cylinder is aluminium, seems adequate
Cylinder head is plastic
Piston is plastic (half of the teeth are metal)
Piston head is aluminium - I believe this is where the metal shaving on the O-ring came from as the edges inside some of the ports are a little rough & I was able to remove some more shavings from them.
Gears look decent.
Gearbox uses 8mm bearings, they look fine.
The stock shimming and motor height adjustment was good.
The application of grease seemed a little thin & it felt more oily than greasy.
I didn't pay much attention to the MOSFET & all I can say about that is it seems basic but appears to work well enough.
The motor feels highish torque; the magnets seem fairly strong and it's much harder to turn by hand than the "standard" motors that are in my other guns. It sounds a bit whiny though so I might replace it at some point.
The only parts that I feel really needed to be swapped out are the hop rubber & nub, plus maybe the spring depending on the desired power level. The rest is fine with just a clean & regrease at most.
I wanted it to be somewhere around 330-340 though so I also replaced the stock cylinder with a Guarder chromed cylinder, the piston head was replaced with a Lees Precision aluminium piston head (I reused the stock piston O-ring though - I find the LPE piston O-rings tend to be a bit tight fitting & give lower fps) and I regreased the gearbox using Abbey LT2 and TechT Gun Sav. I also replaced the mini-Tamiya connector with a Deans connector.
With this work done it now performs how I want it - using the stock spring the power rose to a little under 330fps; with a Guarder SP90 spring it does a bit over 330fps.
So mine seemed decent enough for the most part (aside from that dodgy hop rubber and piston head), but I can't say if they've improved things recently or I was just lucky, so YMMV.