Actual 7.62 NATO velocity loss - 21" bbl to 16" bbl

[iamkris]

I went to the range today to finally play with some Christmas toys I got...sighted in the Millett DMS-1 1x-4x illum reticle scope on my Para FAL, played with my new Shooting Chrony and tested some handloads.

What a great time, I thought, to find out actual velocity readings with exactly the same loads, shot from a 16.25" bbl FAL back-to-back with a 21" StG58A FAL. I've heard lots of people say that you should expect 50+ fps per inch of barrel velocity loss...I wanted to see what two back to back rifles actually would yield.

It was very cold at the range (-2 deg F) so I only shot 5 rounds for each load in each rifle. Also, the overall velocities for all loads were lower than I expected...I'm assuming that is a temperature effect. These readings are averages of 5 shots each read from the Chrony.

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

Now this isn't apples to apples since it isn't the same rifle, same barrel and only difference being the length. For the statisticians out there, the sample size is rather small as well, both in number of rounds fired as well as number of firearms. Other factors, like bore size, wear, chamber size, etc, can influence velocities.

That said, with these two rifles, you can see that the delta is in the 120-140 fps for 5 inches of barrel. That puts the velocity loss at 25-30 fps per inch of barrel.

You can make your own judgements if that matters to you. Here's the rifles

 

[351 WINCHESTER]

Nothing less than 18.5 inches for me. The muzzel blast from a 7.62 nato from a carbine is pretty bad.

 

[LoadedDrum]

Was the gas setting the same on both rifles?

 

[iamkris]

Was the gas setting the same on both rifles?

Good question...not the same but the rifle is on 5, the carbine on 4. Next time I'm at the range I'll check to see what the velocity loss shot-to-shot is as you change the gas valve setting.

The muzzel blast from a 7.62 nato from a carbine is pretty bad.

That hasn't been my experience. I don't find any difference in the blast from behind the rifle between a 21' and 16". It may be different from the sides though...I'm not really worried about that.

 

[spiroxlii]

My only rifle chambered for 7.62 NATO is my Ishapore 2A, and its barrel is anything but short.

To eliminate some of the variables, it would be interesting to see this test done with two bolt action rifles that were nearly identical in all ways except barrel length. Maybe a full size Ishapore 2A and one of the Ishapore 2A rifles that has been cut down to carbine length to look like an Enfield No. 5.

 

[Ian]

A simpler way to control for gas port size would be to simple put both FALs on the grenade setting, where no gas goes into the operating system. Then you basically have two straight-pull bolt actions.

It would also be interesting to try a few more barrel lengths, to see the curve of the velocities. Maybe there's an ideal intermediate length, and I wonder where the point of diminishing returns is as you get longer?

 

[Javelin]

I would be interested to see if an 18" barrel makes the difference. Does anyone have the information on how long the barrel needs to be to achieve maximum velocity for a 7.62? I have been shopping for the perfect AR-10 myself and cant decide on what I want. 16" looks promising but with these results I cant see going that short.....

 

[SnakeEater]

Thank you for doing the test. Unless the intended purpose of the rifle is MAX distance shooting, the velocity loss is basically a moot point. M1A's, FALS, AR-10's are not rifles suited for MAX distance shooting anyway.

I have been shopping for the perfect AR-10 myself and cant decide on what I want. 16" looks promising but with these results I cant see going that short.....

In 3 weeks my bonus check will arrive and I have plans to use it up. I'm leaning towards the Noveske 18" with S&B "Short Dot" right now. Yes, I know spending $5K+ on a rifle and scope is sorta silly, but I don't care.

--18" Heavy 7.62mm stainless barrel, 1 in 10" twist
--7.62 Noveske Match Mod 1 chamber
--14" railed handguard
--Mid length gas system, 7.62mm
--Extended feed ramps
--5/8x24 threads
--.875" low-profile gas block pinned to barrel
--Beadblasted finish
--Smith Vortex flash suppressor
--Properly staked carrier key
--Mil-Spec receiver extension, staked
--H3 Buffer
--Vltor Carbine buttstock
--Tango Down pistol grip
--One 20 rd mag included
--Flip-up front and rear sights
$3328.98

 

[Slamfire]

Good and interesting data. Glad you posted it. Love your FAL's.

Gas setting is not going to make much of a difference in velocity. But the barrel length and internal dimensions of the barrel will make one heck of a difference on velocity.

Those short barrels, they make one heck of a blast. The worst I have used is the British Jungle Carbine and those M44 Nagants. Like a fireball in your face.

 

[Javelin]

In 3 weeks my bonus check will arrive and I have plans to use it up. I'm leaning towards the Noveske 18" with S&B "Short Dot" right now. Yes, I know spending $5K+ on a rifle and scope is sorta silly, but I don't care.

In in our economy that must be nice to have those resources.

 

[SnakeEater]

Honestly Javelin, the economy is the reason I'm doing it. I don't think $5K will be worth very much in the not to distant future. Might as well spend some now while it still buys things. IMHO things will start to really spiral out of control in 2 weeks. Have you seen Jim Cramer's predictions?

 

[Father Knows Best]

Great data, kris. Thanks for posting.

As for muzzle blast, I have a 16.25" barrel Para FAL and an 18" barrel PTR-91. I can't tell any difference in muzzle blast between them. I've also fired a 21" barrel FAL quite a bit -- no noticeable difference to me.

On the other hand, my dad's shop has an 11" barrel FAL (DSA SA58 OSW). The blast from that thing is extremely unpleasant. I sure wouldn't want to fire it indoors.

 

[Nightcrawler]

The velocities all seem low. My carbine chronographed over 2,500 FPS to the low 2,600s with South African, which runs a bit milder than standard NATO ball in my experience. Averaging 2500-2600 feet per second from a 21" barrel seems a little on the slow side to me. Are these handloads using surplus bullets, or actual surplus ammunition?

Hmm....seems to me that if you're going to handload anyway, you could tailor the loads to get the most out of the 16" barrel, like guys with T/C pistols do. Just a thought.

Looks like about a hundred to a hundred and fifty feet-per-second loss. Not bad for losing five inches of barrel.

 

[iamkris]

The velocities all seem low. ...Are these handloads using surplus bullets, or actual surplus ammunition?

Just testing out some handloads...I didn't know if they'd be up to NATO spec or not. Looks like...not.

Then again, it was VERY cold today. I don't know how much temp will affect velocities...I've certainly heard high heat increases pressure and thus velocity.

you could tailor the loads to get the most out of the 16" barrel

That's a great idea...this test was just with IMR4895. I might try a slightly faster burn rate powder.

 

[iamkris]

This is interesting...here's a writeup from Sierra on temp effects of interior ballistics. They state

(1) A cartridge has a muzzle velocity sensitivity to cartridge temperature of about 1 fps per degree Fahrenheit. These are only approximate, and they apply only for the moderate loads used.

http://www.exteriorballistics.com/ebexplained/4th/56.cfm

At the temps today, we were around 60 deg F below standard day temps. Don't know if their test can extrapolate to my experience or not. Loads #3 and #4 "feel" like surplus ammo...add 60 fps to their velocities from a 21" barrel and it seems pretty close to the 2650 fps standard.

 

[eliphalet]

Interesting iamkris.
Here's some data on .223 with several powders, in progressively shorter barrels using the same gun.

http://www.accuratereloading.com/223sb.html

 

[spiroxlii]

Cartridge temperature may not be the same as ambient temperature after your first few shots. It might be 30 degrees outside, but inside that chamber after a full mag, it's quite toasty. If we're talking about the effect of cartridge temperature on bullet velocity, then that's something to consider.

If the change in bullet velocity actually has more to do with air density at colder temperatures, then that's different. Anybody who knows more about this than I do want to chime in?

 

[Nightcrawler]

I think the standard NATO loading is a 147 grain bullet at about 2,750 FPS, actually. It can actually vary noticably depending on which country is making the ammunition and what year it was made. Then, of course, you have commercial .308, which typically runs a little hotter. Winchester lists their 147 grain FMJ at 2,800FPS. That's most likely from a 24-26" barrel, though.

I'd be really interested to see what kind of short-barrel load you could work up to maximize a 16" .308. If you could consistently get 2,700FPS from a 16.25" tube without excessive pressure you'd be spot on, I think. (Plus, there's no better military rifle than FAL to handle hot loads; just turn down the gas system and vent the excess gas pressure!)

 

[iamkris]

Cartridge temperature may not be the same as ambient temperature after your first few shots. It might be 30 degrees outside, but inside that chamber after a full mag, it's quite toasty.

True, but in this case, my range is a "only one round in the rifle at all times...no full mags". So that doesn't apply in this case.

standard NATO loading is a 147 grain bullet at about 2,750 FPS

Next time I have these girls at the range with the Chrony, I'll run a little test with SA and Port.

 

[iamkris]

Could someone validate my logic? If I wanted to get a higher velocity load than with IMR4895 in a 16" barrel, but still within safe pressures, does it make sense to go with a slightly faster burning powder? If so, I'm thinking best to start with IMR3031 or Reloader 7 (which I have on hand).

Powder burn rates
http://groups.msn.com/SecondAmendmentfriendswithguns-/gunpowderburnratechart.msnw

 

[Nightcrawler]

I'll be very interested to see the difference between South African R1M1 and Portuguese NATO stuff. When I used up the last of my Port a few years ago and switched to South African, I had short cycles and had to crank up the gas system from 5.5 to 4. It felt like the South African was running at a lower velocity; it seemed to recoil less, too.

I'm curious if there's anything to that or if it was all in my head.

One thing I love about the .308 is the ammo variety available. I don't have the excess cash to go around testing things, but you've got everything from 110 grain varmint loads to 180 grain elk loads, not to mention weird things like the SABOT Accelerator rounds. (A .308 SABOT-ed down to a .223 bullet.)

 

[iamkris]

When I used up the last of my Port a few years ago

I've still got 3K rounds sitting down in the basement, stockpiled from when you could buy it for $119 from AIM. Everytime that little red guy with the horns and pitchfork standing on my shoulder makes me start to touch it, the other little guy with the harp convinces me to save it.