Your plan is fine if that's what you want to do, but I wouldn't really classify what you're proposing as really having much, if anything to do with the HK416/M27 other than barrel length.
The long and the short of it is that not all piston systems are the same, nor are all piston guns equal. Some might have varying opinions on whether the HK416 system is better or worse than any other system, but the HK416/M27 is different from a standard AR in more than just the piston, that it is external piston driven is, IMHO, more incidental of the particular weapon, than the sole reason for its success (at least in terms of military procurements).
Stated another way, what you're saying is basically saying that a Glock = an M&P = and XD--they're all striker fired, but they're not the same, so things that hold true for one might not hold true for the others besides the basic fact of their operating principle.
~Augee