I'm betting Geissele went with longer "NATO" type throats for reliability.
With Proof CF barrels and an older lot of Hornady 108 ELD-M:
14.5" 2560 FPS
18" 2660 FPS
Current batch is
a lot slower in 18". Haven't tried it in the 14.5".
The older lot was also noticeably temperature sensitive. One degree F gave a 2 FPS difference. Have not had the chance to try warmer temps with the new lot.
Here are some recent velocity/SD numbers (10 plus round sample size) with
brand new Proof CF barrels shooting Hornady 103 ELD-X:
18" 2627 FPS / SD 12.2 FPS
18" 2629 FPS / SD 10.6 FPS
16" 2568 FPS / SD 13.4 FPS
All were sub-MOA, with one 18" breaking half-MOA in the last of three groups and the 16" going half-MOA on the second group. That's with a 75 year old wearing trifocals shooting off a bipod.