Optimal Barrel Length

[For Gallantry]

I built my Grendel about a year back with a 20" Shilen barrel. While accuracy is outstanding, my velocity numbers over my Magneto Speed are well short of what I had hoped for. I see most of the book data was derived from a 24" tube and while browsing around I see that Satern offers a 28" barrel. I'm wondering if anyone has any firsthand experience with such a long barrel in this chambering and some real-world velocity numbers. Threads on other sites are all over the map-some saying that one may realize about a 60 FPS gain over a 24" barrel while others say you'll actually lose velocity. I understand the 28" is A LOT of barrel but the primary use for this gun is banging steel at long range and punching paper. Thanks in advance.

[grayfox]

When you say your "velocity numbers... are well short" what kinds of MV are you getting for which bullet/powder/primer/brass loads? What distance are you looking for, at what elevation ASL?
I'm not a long-barrel shooter, but for a small-case style cartridge like the Grendel, you might get no real increase or lose MV after 24-26"...
Others with more long-barrel experience might chime in.

[VASCAR2]

There is a web site for velocity prediction plus a graph that has been posted on the forum showing the velocity trends for various 6.5 Grendel length barrels. Generally once you get past 24” with the 6.5 Grendel the velocity increase per inch is pretty small. Bwaites has a 28” 6.5 Grendel if I remember correctly and maybe he or another member will post their chronograph results. From the results below going from a 20” to 24” with 123 grain A-Max yeilds a 63 FPS increase or about 15 FPS per inch. With shorter barrels the increase in velocity loss per inch is more noticeable on barrels below 20”. With the 6.5 Grendel small case capacity the drag/friction from the barrel degrades some of the velocity from longer barrels.

I copied my chronograph results which I previously posted.


This question comes up fairly frequently so a couple years ago a Friend and I chronographed 4 different 6.5 Grendel AR-15’s. The temperature was approximately 70 degrees and we used a Magnetto speed chronograph. We chronographed several loads in a 16” J&T Dist/Double Star 1:9 twist CMV, 18” 1:8 twist SS Grendel II Satern/Liberty, 20” Shilen SS 1:9 twist and a 24” AA 1:9 twist CMV. Below are the results. These results were obtained prior to PPU recalling their 6.5 Grendel ammo. All ammo tested was out of the same lot to help in getting consistent comparison.

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 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



Link to velocity calculator site.


http://shootersnotes.com/Vel65Gren.php

[biodsl]

Forum user bwaites has a 28" barrel. This will get you started

http://www.65grendel.com/forum/showthrea...ght=barrel

[For Gallantry]

When you say your "velocity numbers... are well short" what kinds of MV are you getting for which bullet/powder/primer/brass loads?

2245 FPS from a 120 grain TTSX over 27.0 grains of RL15 in Lapua brass...book velocities are over 2500 FPS

[VASCAR2]

Here is the velocity calculator graph.

49B5F7E7-B98B-4324-A30A-66BACBA3A740

[Bigs28]

Try a different powder?

[VASCAR2]

2245 FPS from a 120 grain TTSX over 27.0 grains of RL15 in Lapua brass...book velocities are over 2500 FPS

Hornady list max charge of 29.0 grains Reloader 15 with 120 to 123 grain bullets. Looks like you could get a little more velocity out of your barrel by going to a higher charge or different powder. Typical velocity with a 120 grain bullet in a 20” 6.5 Grendel is about 2475 to 2520 FPS from my testing.

[grayfox]

Hornady list max charge of 29.0 grains Reloader 15 with 120 to 123 grain bullets. Looks like you could get a little more velocity out of your barrel by going to a higher charge or different powder. Typical velocity with a 120 grain bullet in a 20” 6.5 Grendel is about 2475 to 2520 FPS from my testing.

+1 ^^^ what he said.
Also with the longer barrel you could get some more ooomph with a slower powder... like maybe cfe123.
Seems like a 20-22-24" barrel would get you what you might be looking for.

One more thing, the 120 ttsx with a G1 of 0.412, doesn't have what I would call a 1000-yd BC... for bangin' steel why not go to a better-BC bullet. 129 Ablr, Hdy 123-anything, lapua scenar's, sierra SMK's.... lots of 1000-yd choices.

[muvef]

RL15 is slow. Your numbers sound typical.