velocity out of 16 inch barrel is just not that much slower

[Brenten]

So on another thread here someone was asking about 16 inch barrel reach.  I googled and came up with some very tall tales on another forum (THR).  Some were saying 100ft/ser per inch lost, some were saying, 75 and 50. My experience has been that above 16 inches, the difference in velocity has not been much, 20-30 fps max.  So I figured I would look up some old actual tests on the forum and put them in here to get an average of what reality is, not tall tales and opinions. 

First up is this:

Temp 72

Alt 5200

.................DSA SA58 16" bbl..........FN PBR XP 20" bbl

American Eagle .308Win 150gr FMJBT #AE308D

...........................2538................... ........2669

...........................2537................... ........2681

...........................2506................... ........2703

...........................2538................... ........2635

.....................Avg 2543.....................Avg 2663

....................SD......32.23................S D.......32.81

120ft/sec difference

Federal .308Win 150gr Soft Point #308A

...........................2588................... ........2724

...........................2594................... ........2741

...........................2595................... ........2729

...........................2582................... ........2736

...........................2605................... ........2726

.....................Avg 2593.....................Avg 2731

....................SD........8.54................ SD........7.07

138ft/sec difference

Barnaul .308Win 168gr FMJBT

...........................2424................... ........2554

...........................2388................... ........2515

...........................2455................... ........2507

...........................2379................... ........2505

...........................2399................... ........2518

.....................Avg 2409......................Avg 2520

....................SD......30.74................S D......19.92

111ft/sec difference

This came from gun end game.  The difference in the 4 inches was 120 average or 30 fps per inch.

next up is:

Member

Join Date: November 16, 1999

Location: Phoenix, AZ

Posts: 28

OK folks, here's the facts. I have a 16" FAL (built by Arizona Response Systems) and a 21" L1A1 by Century. Chronograph numbers:

Radway Green:

21" FAL - 2693fps (30 shot average)

16" FAL - 2645fps (10 shot average)

Portugese FNM:

21" FAL - 2663fps (5 shot average)

16" FAL - 2599fps (5 shot average)

from firing line.  This difference ended up being 11 fps per inch.

Next up is:

I'm assuming you are advocating 18" for velocity reasons, here's a breakdown from KevinB of Knight's with velocity from 16" EMC and 20" SR25.

16" M110 Carbine

Average Muzzle Velocity (of 10 Rounds) in Feet per Second (FPS) in 16” Barrel M110 Carbine:

Ammunition Un-Suppressed

M118 LR U.S. Army - 2475

Lapua 167 gr. - 2465

Lapua 170 gr. - 2478

Lapua 185 gr. - 2265

Average Muzzle Velocity (of 10 Rounds) in Feet per Second (FPS) in 20" M110 SASS

M118LR - 2556

Lapua 167gr - 2624

Lapua 170gr - 2637

Lapua 185gr - 2410

In this one I took the 170 grain as a ball park and it gave a difference of 40 fps per inch, however you will note that this rifle shot much higher velocities than the other 308's.  Better tuned gas action most likely. 

next up is:

Load #1: 41 gr IMR4895, 150 gr surplus FMJ, 2.80" OAL

DSA 16.25" Para FAL 2311 fps

DSA 21" StG58A FAL 2449 fps

Load #2: 42 gr IMR4895, 150 gr surplus FMJ, 2.80" OAL

DSA 16.25" Para FAL 2379 fps

DSA 21" StG58A FAL 2501 fps

Load #3: 43 gr IMR4895, 150 gr surplus FMJ, 2.80" OAL

DSA 16.25" Para FAL 2453 fps

DSA 21" StG58A FAL 2598 fps

Load #4: 44 gr IMR4895, 150 gr surplus FMJ, 2.80" OAL

DSA 16.25" Para FAL 2501 fps

DSA 21" StG58A FAL 2643 fps

from THR. The guns were two DSA Stg58's.  138, 122, 145 and 142 fps were the differences.  The average was 137 fps/ 4.75 inch barrel length difference gives 29 fps per inch. 

From G&A combat arms issue of regarding an 11 inch barreled DSA, the author tested velocity of Black hills 175 grain match out of the 11 inch DSA was 2,180 fps.  The same load out of a 21 inch FAL was 2523 fps.  The loss per inch was 34 fps.  Loss from inch to inch isn't linear and I am sure that the differences in the shorter barrels is greater than the longer barrels so this would be a worst case.

If you take average difference per inch of each of the above cases you get  (30+11+40+29+34)/5= 30 fps per inch. 

I know this is not perfect, but definitely better than most WAG's. 

enjoy

[TacBlade]

Great Info!!!  Thanks!!!

[98Z5V]

That is some great info!  I think the one thing that really jumped out and got me was that the KAC tests from KevinB included a Lapua 185gr round, and he fired 10 rounds - I'm assuming he loaded those to mag length and fed them through a KAC mag.  I'd love to know the OAL he loaded them to, and how he got them to that length with the 185gr bullet.  That is sweet!  <thumbsup>

[Brenten]

I bet they load the 185's routinely both regular and subsonic.  One thing I noticed is how hot the lapua ammo is out of the KAC.

[SWshooter]

There are a number of factors to think about with barrel length. A 200 fps difference in velocity is significant, bullet energy increases as the square of the velocity. Muzzle blast in a really short barrel can be literally deafening if you have to fire without ear protection, I'm talking permanent hearing loss. Muzzle flash can be blinding in dim light. Loads that are accurate in a long barrel might be unstable in a shorter barrel. SBR's in particular can be dangerous, the short overall length makes it easier to shoot yourself or a buddy, and contact and flash burns are more common because of the closeness of the muzzle to your left hand.

[Brenten]

200 fps? where did you get that?  at best there is a 120 fps difference between the 16 and the 20 inch.  Not much of a difference and definitely not worth the extra length, Unless you are planning 1000 yard shots.  As for noise, any non silenced rifle will cause damage.  An M4 with it's 14 inch barrel and rounds leaving the barrel at 3200 fps is much worse than a 308 traveling 500 fps less and guess what, that M4 is the most common assault weapon by gov/le/military. I have shot both short barreled 308's and short barreled 223's and the 223's are much more violent.  My M4 with 14.5 barrel is much worse than my POF 308 11.5 inch barrel.

The point of what I posted was to show reality about the differences in velocity. 

[SWshooter]

The point of what I posted is to show there are other concerns besides velocity with short barreled rifles. Velocity is probably last on the list of reasons to shorten a barrel. I agree that the 556 is worse than the 762 for muzzle blast. The larger the caliber the less the muzzle blast, generally. I was comparing SBR's to 20" rifles, and I'm sure you will find a 200 fps difference between an 11" barrel and a 20" barrel. If I were making an SBR I would look at a 7.62x39 as a much better cartridge than either a 5.56 or 7.62 NATO. You could build it on the smaller receiver, and the larger bore size would have less flash and blast. The heavier, fatter bullet would be a more effective combat cartridge. Looking at the posted data shows a 343 fps difference between an 11" and 21" barrel, with 175 gr match bullet. That is certainly significant. Generally the heavier bullets are more efficient in short barrels as the slower bullet has a longer barrel time and burns the powder more efficiently.  I would certainly not use the governments choice in weapons as any kind of recommendation. The procurement process is more about politics than providing soldiers with good weapons.

[Brenten]

I am limited to using an M4 for work.  But if I had a choice between a 7.62x39 or 308 I would choose the 308 as it offers much more penetration even in an 11 inch platform regardless of the size.  An 11 inch 308 has so much more power it isn't even close.  But you are right the velocity loss going to 11 inches is closer to 200 or maybe more.  Still the knockdown power of the 308, even in the short 11 barrel is incredible. 

[J.Boyette]

Guys,

I run a factory DPMS 16" LR-308 with a 1/10 twist.

I have shot it to 700yds, but mostly 600yds and less. What I have found works best is to shoot a lighter bullet like a 155grn match grade with a faster powder like W748 and mag primers I get loads that are able to perform like a longer barrel rifle.

Just an idea.

[Brenten]

makes sense.  I think that the major ammo makers will be doing this as well to accommodate all of the quality short barreled platforms.