An update, of sort.
I've skirmished with the weapon once, accuracy was horrible.
Upon checking the hop-up units, the barrel was very dirty and packed with dry grease.
Hop-up rubber sleeve was crap.
Hop-up nub was not perfect either, not perfectly round or well-cut.
The barrel of the HK416 (full stock, 14.5'' version) is close to 400mm - longer than your standard M4 barrel, but still WAY shorter than the M16 barrel I believe.
Not luck finding a Vernier Caliper, but would be extremely intersting if someone can measure out the diameter off the barrel.
Things I replaced:
1) Guarder hop-up sleeve
2) Element hop-up nub (Guarder nub was too hard, over-hop-up even with hop off)
And that's it
Overall, very impressive package, but needs some maintenance work before it's field ready.