I think it helps to have a velocity comparison for the different barrel lengths. If you have the velocity and Ballistic Coefficient you can calculate the down range velocity and trajectory for the different bullet weights and barrel lengths. Here are some chronograph results Ive gotten plus some 12 chronograph data posted online. For most 6.5 Grendel bullets to expand you need between 1800 to 2000 FPS. One exception is the Nosler 129 grain Accu Bond Long Range (ABLR) which supposedly expands down to 1300 FPS. A couple of forum members have used 12 6.5 Grendel to take Deer. It is surprising how well the 6.5 Grendel can perform out of shorter barrels.
Barrel length can give more velocity but a longer barrel doesnt mean it will be more accurate. In the current buying frenzy you might have better luck trying to assemble an upper compared to buying a complete upper. The answers you seek are here on the forum. Hopefully the chronograph results will help you get an idea of the 6.5 Grendel velocities and which barrel length meet your needs. The shorter barrels are handy in dense foliage and the longer barrels are nice in more open terrain. I think the 18 length 6.5 Grendel hit the sweet spot for performance and maneuverability. In an AR-15 I think the 24 and longer barrels are not practical except from a bench, prone or hunting blind.
12 6.5 Grendel chronograph results
Buffman Range video 12 PSA 6.5 Grendel Federal Berger 130 grain OTM GMM 2073 FPS (Pro Chrony)
18 PSA Nitride 6.5 Grendel Federal 130 grain Hybrid-OTM Berger GMM 2322 FPS (Pro Chrony)
12 PSA SS with LabRadar.
123gr Hornady American Gunner bulk HPBT = 2201 FPS average with SD of 14.7 (2" to 2.55" groups)
123gr Hornady ELD-M Black = 2265 FPS average with SD of 13.2 (1.4 to 1.7" groups)
123gr Hornady Custom SST Factory= 2277 average with SD of 11.8 (2.9 to 3.2" groups)
12 Faxon
Wolf steel case at 2364 Avg. SD was 9.7.
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
12 2265 (123 grain ELD-M)
16" 2419
18" 2464
20" 2505
24" 2568
123 Grain SST Hornady
12 2277
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
12 2364
16" 2507
18" 2571
20" 2615
24" 2695
Hand Load 107 Grain SMK 28.5 IMR8208XBR, CCI 41 Hornady cases.
20" 2658
24" 2729
Hornady 123 grain American Gunner
12 2201
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
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
Difference between 24" and 20"
PPU 120 grain HB BT
79 FPS
PPU 110 grain FMJ
69 FPS
123 grain A-Max, Hornady
63 FPS
123 grain SST, Hornady
72 FPS
Hand Load 123 grain Nosler CC, 31.0 grains CFE223, CCI 450
93 FPS
100 grain FMJ Wolf
80 FPS
Hand Load 107 grain SMK 28.5 grains IMR8208XBR, CCI 41
71 FPS
Barrel length can give more velocity but a longer barrel doesnt mean it will be more accurate. In the current buying frenzy you might have better luck trying to assemble an upper compared to buying a complete upper. The answers you seek are here on the forum. Hopefully the chronograph results will help you get an idea of the 6.5 Grendel velocities and which barrel length meet your needs. The shorter barrels are handy in dense foliage and the longer barrels are nice in more open terrain. I think the 18 length 6.5 Grendel hit the sweet spot for performance and maneuverability. In an AR-15 I think the 24 and longer barrels are not practical except from a bench, prone or hunting blind.
12 6.5 Grendel chronograph results
Buffman Range video 12 PSA 6.5 Grendel Federal Berger 130 grain OTM GMM 2073 FPS (Pro Chrony)
18 PSA Nitride 6.5 Grendel Federal 130 grain Hybrid-OTM Berger GMM 2322 FPS (Pro Chrony)
12 PSA SS with LabRadar.
123gr Hornady American Gunner bulk HPBT = 2201 FPS average with SD of 14.7 (2" to 2.55" groups)
123gr Hornady ELD-M Black = 2265 FPS average with SD of 13.2 (1.4 to 1.7" groups)
123gr Hornady Custom SST Factory= 2277 average with SD of 11.8 (2.9 to 3.2" groups)
12 Faxon
Wolf steel case at 2364 Avg. SD was 9.7.
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
12 2265 (123 grain ELD-M)
16" 2419
18" 2464
20" 2505
24" 2568
123 Grain SST Hornady
12 2277
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
12 2364
16" 2507
18" 2571
20" 2615
24" 2695
Hand Load 107 Grain SMK 28.5 IMR8208XBR, CCI 41 Hornady cases.
20" 2658
24" 2729
Hornady 123 grain American Gunner
12 2201
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
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
Difference between 24" and 20"
PPU 120 grain HB BT
79 FPS
PPU 110 grain FMJ
69 FPS
123 grain A-Max, Hornady
63 FPS
123 grain SST, Hornady
72 FPS
Hand Load 123 grain Nosler CC, 31.0 grains CFE223, CCI 450
93 FPS
100 grain FMJ Wolf
80 FPS
Hand Load 107 grain SMK 28.5 grains IMR8208XBR, CCI 41
71 FPS