24" Barrel lightweight AR build?

[VASCAR2]

Here is some chronograph data I got using a Magnetospeed a few years ago and tdbru is correct. The velocity gain per inch decreases more past 20”. The shorter the barrel the more FPS are lost per inch but with longer barrels once you get passed 20” the gain in velocity is less per inch. There is also a graph illustrating the velocity for given barrel length.

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