12-21-2018, 06:23 AM
The main difference will be velocity is a little higher with the 20" barrel compared to 18" barrels. The higher velocity gives you a little more distance before the bullet drops below expansion threshold. A friend and I chronographed four 6.5 Grendel AR-15's with a Magnettospeed chronograph a couple years ago. The barrels were a 16", 18", 20" and 24".
Here was the difference in average velocity difference 18" and 20" barrels.
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
Here was the difference in average velocity difference 18" and 20" barrels.
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