20” is already very short in my book. I very much doubt that any tactical advantage of losing 7” of barrel makes up for the expense and logistics tree of having that option. And with any gas operated gun, barrel length changes affect operation. Yes ports can be changed, but that is a compromise, not an optimization.
In the tests that I have seen regarding the 16" barrel on the SOCOM M1A, the performance of the .308 through that length barrel was similar to 300 Savage (150 gr at 2600fps). The big tradeoff for the handiness of the shorter package was significantly increased muzzle blast and noise. And who is giving up 7" of barrel length? The SCAR has a 20" barrel option. The M14 barrel is 22" long.