A Friend and I have four 6.5 Grendel AR-15's between us and he also owns a Magnetospeed chronograph. These velocities and Ballistic calculator should give you an idea of the 6.5 Grendel performance. There is a graph showing approximate velocity loss or gain for different barrel lengths if you go shorter than 16" or longer than 24".
Here is a list of the tested ammo by average velocity for each barrel length.
120 Grain HP BT PPU
16" 2392
18" 2455
20" 2480
24" 2559
110 grain FMJ PPU
16" 2510
18" 2570
20" 2614
24" 2683
123 grain A-Max Hornady
16" 2419
18" 2464
20" 2505
24" 2568
123 Grain SST Hornady
16" 2430
18" 2493
20" 2511
24" 2583
Hand Load 123 Grain Nosler CC, 31.0 grains CFE223, CCI 450
16" 2449
18" 2471
20" 2526
24" 2619
100 Grain FMJ Wolf
16" 2507
18" 2571
20" 2615
24" 2695
Hand Load 107 Grain SMK 28.5 IMR8208XBR, CCI 41 Hornady cases.
20" 2658
24" 2729
Here is a list of the tested ammo by average velocity for each barrel length.
120 Grain HP BT PPU
16" 2392
18" 2455
20" 2480
24" 2559
110 grain FMJ PPU
16" 2510
18" 2570
20" 2614
24" 2683
123 grain A-Max Hornady
16" 2419
18" 2464
20" 2505
24" 2568
123 Grain SST Hornady
16" 2430
18" 2493
20" 2511
24" 2583
Hand Load 123 Grain Nosler CC, 31.0 grains CFE223, CCI 450
16" 2449
18" 2471
20" 2526
24" 2619
100 Grain FMJ Wolf
16" 2507
18" 2571
20" 2615
24" 2695
Hand Load 107 Grain SMK 28.5 IMR8208XBR, CCI 41 Hornady cases.
20" 2658
24" 2729