Most Popular Barrel Length for 6mmARC?

[6mmAR]

I'm gonna say 18"... Only because that's what I have and it seems that's the length it was supposedly designed around?

Hmmm - - You may be and probably are right, but I doubt one will achieve the touted 2750 fps out of the Hornady "Black" 6mm ARC ammo with the 105 gr bullet out of that length barrel.  FWIW I also doubt one would get there with my suggested 20" barrel either.  With my work with the 6mmAR, we found that velocity seemed to peak around 25", although there was very little increase between 24" to 25".  The proven method for determining this was by taking a couple 28" barreled uppers and a hack saw with a hand crowning tool with the chronograph (as well as plenty of weighed charges ammo) to the range and chronographing 5 shots, then saw and inch off the barrels, then go again, then saw and inch off, etc.  That method really gives true data on barrel length vs velocity.