On an 18 6.5 Grendel the rifle length gas system makes a significant difference if your going to use a suppressor. I have an 18 Group Buy Faxon Monster logo barrel and it is perfectly gassed using a supplied carbine buffer and spring with my collapsable stock. Here is some chronograph data I collected using a Magnettospeed chronograph comparing the velocities between 16 & 18 6.5 Grendel. This data is a few years old but the trends in velocity still hold true with current ammo. The 16 barrel was a J&T Distributing (sister Co. to Double Star) and a Brownells 18 rebranded Satern button rifled barrel. For a hunting barrel I dont think you could go wrong with either Lilja or Rainier.
Here is the difference between the 18 barrel compared to the shorter 16 barrel which was slower with every load.
120 grain PPU HP BT
63 FPS difference
110 grain PPU FMJ
60 FPS difference
123 grain A-Max
45 FPS difference
123 grain SST
63 FPS difference
Hand load 123 grain Nosler CC
27 FPS difference
Wolf 100 grain FMJ Steel cased
64 FPS difference
Here are the velocities for factory 123 grain SST we got in our velocity test with the various barrel lengths.
123 Grain SST Hornady
12 2277
16" 2430
18" 2493
20" 2511
24" 2583
Here is the difference between the 18 barrel compared to the shorter 16 barrel which was slower with every load.
120 grain PPU HP BT
63 FPS difference
110 grain PPU FMJ
60 FPS difference
123 grain A-Max
45 FPS difference
123 grain SST
63 FPS difference
Hand load 123 grain Nosler CC
27 FPS difference
Wolf 100 grain FMJ Steel cased
64 FPS difference
Here are the velocities for factory 123 grain SST we got in our velocity test with the various barrel lengths.
123 Grain SST Hornady
12 2277
16" 2430
18" 2493
20" 2511
24" 2583