I have an 18.5" barrel that shoots 175gr Gold Medal Match faster than a 24" factory proof barrel. Data comprised of a single comparison doesn't make for good generalizations. I'd be interested to hear the data on what your 24" bolt gun runs the same ammo (FGMM 175gr) over the chrono, I'm curious to see if your theory about barrel length and velocity holds water. 2610fps for a 175gr SMK out of a 18.5" 1:10 twist barrel, averaged over a 100-shot string, 70° ambient temperature >10% humidity.
I think you're vastly overestimating what your gas losses are, and I'm 100% positive that barrel length won't have enough effect on your velocities to otherwise negate your effectiveness at the ranges you discussed. Don't take my word for it though, build your system and prove your data, or disprove it, either way, we all learn and benefit.