Back to the topic. . .
The reason a shooter might ask this question is (as previously discussed) to determine the gas system and barrel length combination that would allow minimal recoil, while still harvesting enough gas to create a strong, reliable, COMPLETE bolt cycle.
We all know that 14.5" uppers w/ mid-length gas are reliable and functional. . . ? Right? with an approximate 9" gas block/tube length from barrel to upper, a 14.5" barrel would have ~5.5" of bullet travel beyond the port that is harnessing gas for that length and time of travel.
The rifle length gas system is about 12" in length. When paired with an 18" barrel, that combo would provide .5" more barrel, resulting in ever so little more time that the gas port could harness pressure to cycle the bolt.
SO. . . what you'd have is an upper with a longer sight radius, mild recoil (similar to that of the 14.5" middy) and plenty of time to harvest gas (pressure). Throw in a SS match grade barrel at close to 40 oz and you get perfection balance for a target gun that can get off fast, accurate follow up shots!