This might help but the trend is pretty clear. These results were from a few years ago on a 70 degree day using a Magnettospeed chronograph, five shot strings.
Difference between 20 and 18 barrel 6.5 Grendel
120 grain PPU HP BT
25 FPS difference
110 grain PPU FMJ
44 FPS
123 grain A-Max
41 FPS difference
123 grain SST
18 FPS
Hand load 123 grain Nosler CC
55 FPS difference
Wolf 100 grain FMJ Steel Cased
44 FPS difference
Velocity Difference 20 compared to 24
120 grain PPU HP BT
79 FPS difference
110 grain PPU FMJ
69 FPS Difference
123 grain A-Max
63 FPS Difference
123 grain SST
72 FPS difference
Hand load 31.0 grains CFE223 CCI 450 primer 123 grain Nosler CC
93 FPS Difference
Wolf 100 grain FMJ
80 FPS Difference
Hand Load 107 grain SMK 28.5 grains IMR8208XBR CCI 41 primers
71 FPS Difference
I can post the specific results but they have been posted before. In general going from 20-24 you gain approximately 60-100 FPS higher velocity with the 24 barrel.
Difference between 20 and 18 barrel 6.5 Grendel
120 grain PPU HP BT
25 FPS difference
110 grain PPU FMJ
44 FPS
123 grain A-Max
41 FPS difference
123 grain SST
18 FPS
Hand load 123 grain Nosler CC
55 FPS difference
Wolf 100 grain FMJ Steel Cased
44 FPS difference
Velocity Difference 20 compared to 24
120 grain PPU HP BT
79 FPS difference
110 grain PPU FMJ
69 FPS Difference
123 grain A-Max
63 FPS Difference
123 grain SST
72 FPS difference
Hand load 31.0 grains CFE223 CCI 450 primer 123 grain Nosler CC
93 FPS Difference
Wolf 100 grain FMJ
80 FPS Difference
Hand Load 107 grain SMK 28.5 grains IMR8208XBR CCI 41 primers
71 FPS Difference
I can post the specific results but they have been posted before. In general going from 20-24 you gain approximately 60-100 FPS higher velocity with the 24 barrel.