Actually cutting it down can improve "short range" accuracy (less barrel harmonics), while will degrade "long range" accuracy (less speed makes bullet loose stability faster).
I've seen some sharpshooter rifles that can attest to that. Cutting a .30cal bolt gun down to 16" barrel increases accuracy out to roughly 500 yards. Once you start reaching out into the 800-1000yard range the 20" barrels come back into power. BlackOps Precision makes their rifles almost exclusively with 16" barrels.
EDIT: A lot of the U.S. military experienced lower qualification scores and difficulty qualifying when they transitioned from M16 to M4 length rifles. However, the cause and affect of this was actually the shorter sight radius as opposed to any degradation of accuracy from the barrel itself.
The SCAR does not vent the gas from the gas block/piston, but instead contains it, and then sends it back down the barrel after the bullet exits. HK uses a vented block (apparently with the exception of the 10.5" version), in which the gas pressure in the block releases dependant upon the pistons movement, more than the bullet. There can be such a thing as too short, and a minimum amount of time needed for the gas block to pressurize and operate the piston. However, by design, it shouldn't be as sensitive as the SCAR. For reliability, I can't see HK fine tuning it that far. If anything, the system gets an excess amount of gas, as a precautionary measure. Especially since it will simply vent out any excess, once the piston moves far enough to open the vent hole.
As a little background: I believe I remember reading that Stoner's original direct impingement design needed at least 4-4.5" of barrel after the gas port to pressurize down the gas tube and into the BCG. The amount of time required would still vary by design, but the piston directly in the gas block won't have that delay associated with pressurizing the entire length of the gas tube.
