In this post, Rifleshooter.com will be testing the effects of barrel length on velocity for the 7.62×39 mm Russian Cartridge.
Chambered in the SKS and AK-47, the 7.62×39 mm Russian cartridge is well known throughout the world. Ammunition tends to be widely available (it was first introduced in 1944) and inexpensive, making it an attractive option. In addition to being chambered in imported firearms, the 7.62×39 mm is also available in some domestic rifles like Ruger’s Mini-30 and M77 (on a personal note, two of my first three center-fire rifles were chambered in it, an SKS and Mini-30)!
As a result of its widespread proliferation and popularity, reaching consensus on the specifications of 7.62×39 mm can be a little difficult. Ammunition and firearms for it are manufactured all over the world with varying degrees of quality control. Of particular note is the suggested bullet diameter, with .308″ and .310″ bullets being encountered. The Sporting Arms and Ammunition Manufactures’ Institute (SAAMI), which governs voluntary industry standards for the shooting industry in the United States, suggests a groove diameter of .310″ and bullet diameter of .310″ (-.002″). This means both the .310″ and .308″ diameter bullets will be within specification and can be used.
In this post I’ll be using a Green Mountain chrome moly barrel for the test. The barrel has a .300″ bore, .310″ groove, 1:9.5 twist.
For ammunition, I selected two kinds of Russian ammunition I had on hand, TCW and Brown Bear.
A detailed comparison of the test ammunition can be found in the table below.
The powders and projectiles for both loads are shown below. TCW is on the left, Brown Bear, the right. Note: I have found a few non-annotated sources that indicate TCW is the name Wolf ammunition was imported under before it was called Wolf.
While a FMJ cartridge isn’t represented above, I felt these cartridges represented ammunition that may be encountered by the typical sportsman. It should be noted that 7.62×39 mm ammunition quality can vary greatly, with options ranging from questionably safe, to premium ammunition manufactured by Lapua. The Russian ammunition seemed like a good middle of the road option for testing.
The blank was 1.35″ in diameter and 24″ long (above). I machined the blank into a modified Remington Varmint contour. The barrel was threaded, chambered and installed on a Remington 700 short action receiver (below).
The test rifle looks pretty sharp. This is a picture of it at the end of the experiment with 16.5″ of barrel remaining.
The rifle was built with the following parts from Brownells:
- Green Mountain barrel blank, .310 bore
- Remington 700 short action receiver
- Modular Driven Technologues (MDT) HS3 chassis system
- MDT AICS style magazine
- MAGPUL PRS stock
- Timney 517 trigger
- Harris bipod
The test protocol for this experiment was the same as the other’s we have done (.223, 7mm Rem Mag, 308, and 300 Win Mag). For each barrel length, five rounds of each type of ammunition are fired. Their average velocity and standard deviation are recorded. The barrel is then cut back one inch on a cold saw, and the test repeated.
Velocity data was recorded with a Magnetospeed V3 chronograph.
All data was recorded on the same day. Temperature was 54F.
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A summary of the results for both cartridges is found in the table above. Note the loss in velocity is smaller than one would expect with other cartridges.
The Brown Bear 123 grain HP load lost 97 feet/second as the barrel length changed from 24″ to 16.5″. The maximum velocity of 2454 feet/second was recorded at 23″ barrel length. Average velocity loss was 12.9 feet/second per inch of barrel from 24″ to 16.5″. Average velocity loss was 23.5 feet/second per inch of barrel from 23″ (highest recorded velocity) to 16.5″. Note: For comparison purposes, I recorded ammunition from this same lot at 2326 feet/second, SD 16, from a 16″ Russian AK at 76F.
The TCW 122 grain HP load lost 90 feet/second as the barrel length changed from 24″ to 16.5″. The maximum velocity of 2486 feet/second was recorded at 23″ barrel length. Average velocity loss was 12.0 feet/second per inch of barrel from 24″ to 16.5″. Average velocity loss was 15.8 feet/second per inch of barrel from 23″ (highest recorded velocity) to 16.5″.
Were you surprised by the results?
I had anticipated a greater reduction in velocity when the barrel was cut. I would have guessed 150-200 feet/second velocity loss between 24″ and 16.5″ barrel lengths. It seems like the 7.62x39mm Russian performance is optimum at a 16″ barrel length.
Your data showed the 7.62×39 mm Russian slowed down from 23″ to 24″ in barrel length. Do you think this the case?
I don’t think the data I gathered proved this assumption. The barrel I used was fairly inexpensive and had a rough bore. I fired two rounds out of the barrel to function test the rifle before conducting the experiment. I suspect that the new, rough bore MAY have played a part in the lower 24″ velocities (I noticed extremely heavy bore fouling at the conclusion of the experiment). Also, when you look at the fairly high standard deviations of the cartridges, I don’t know if this sample size supports the assumption that the bullet may have slowed down. If anything, the data showing a decrease in velocity as the barrel increases from 23″ to 24″ raises more questions than it answers. I would suggest further testing at longer lengths with a greater sample size if this interests you.
The 7.62×39 mm Russian is often compared to the .30-30 Winchester, what do you think of this?
I never liked this comparison. The .30-30 Winchester moves a 150-grain bullet with greater sectional density as fast, or faster than a 7.62x39mm moves a 123-grain projectile. I think it is better to compare the 7.62×39 mm to the 300 AAC Blackout (BLK) loaded with 125-grain projectiles. In my testing, in similar barrel lengths, the 300 BLK is within 100 feet/second of the 7.62.39mm with similar weight projectiles. See 300 AAC Blackout Review (300 BLK Review) for more information.
Do you think the MagnetoSpeed chronograph works well?
Yes. I am very confident with its readings and reliability, see MagnetoSpeed V3 versus Oehler 35P: Chronograph comparison and review for more.
How hard was it building a 7.62×39 mm Remington 700?
I built this gun on a standard Remington action with a 308 bolt face, which is larger than the bolt face required. While the gun was safe to fire and fed from a magazine, the extractor had limited engagement on the rim of the fired cartridge resulting in poor ejection. In most cases, the fired case would simply fall out of the bottom of the magazine well. I plan on turning what is left of the barrel into a 7.62×39 mm Remington 700 precision rifle. For this, I will be changing the bolt face to the proper size and installing a Sako extractor.
How did the MDT HS3 chassis work?
It worked like a champ. The HS3 provided a great interface with the pistol and allowed easy removal of the barreled action when I needed to cut it. I believe the HS3 chassis system represents a solid value for the money.
How did the Modular Driven Technology (MDT) magazines work?
The cartridges fed extremely well from the unmodified MDT AICS style magazine.
Did the Remington 700 action reliably fire the 7.62x39mm Russian ammo?
Yes. I fired 92 rounds during the test. 100% of them detonated. I had suspected the “hard” Russian primers would have made firing these rounds difficult. It wasn’t. Note: Two of the TCW cases had minor splits (below), I believe this was the result of brittle necks from the manufacturing process.
Did you just waste a barrel? Ammunition?
No, I don’t think so. I had planned on building a Remington 700 in 7.62×39 mm for a separate post. Since I was able to obtain a 24″ long blank, I decided the first 7.5″ could be used to gather empirical data on the 7.62×39 mm Russian. I had quite a bit of ammo left over from the days when I used to own a AK, so it was expended for a good purpose.
What are possible sources of error in your experiment?
Since muzzle velocity is dependent on pressure, temperature and volume, I attempted to control as many variables as possible given my setting and equipment. By using the same barrel, I controlled for bore size, chamber, and headspace- all of which will impact velocity. Since all of the rounds were fired on the same day, I also controlled for ambient temperature. I did not control for barrel temperature. The barrel did heat up during firing. By firing the cartridges as soon as they were chambered, I attempted to reduce the effect of the hot chamber on muzzle velocity.
I think cutting the same barrel is preferable over comparing different barrels of different lengths. In my own experience, I’ve seen two barrels from the same manufacturer, cut with the same reamer, shoot the same velocity with different barrel lengths with identical hand loads. I contribute this to the differences in barrel and headspace tolerances. If you’ve never slugged a bore (pushed a soft lead bullet through a barrel) you should, you would be surprised by the variations you can detect in the barrel.
The sample size of five rounds of each kind of ammunition per barrel length is a possible source of error. However, my barrel length testing with 308 Winchester indicates it may not be as much as initially thought. I fired 30 rounds of IMI Samson 150 grain FMJ at 28″ and 16.5″ and recorded the results. Comparing the data from the 30 shot strings (28″ 2824 and 16.5″ 2555) to the 5 shot strings (28″ 2823 and 16.5″ 2561) I found a loss of 269 ft/sec (23.4 ft/sec per inch) as the barrel was cut. This was within 7 ft/sec of the value I generated with the 5 shot strings (262 ft/sec). Velocity loss per inch of barrel was .6 ft/sec away (22.8 ft/sec) from the value calculated with 5 shot strings.
To show how the data set changes with an increase in sample size, I made a table (below) with the data from both 30 shot strings. The “shot” column represents the shot number in the respective string. “28” barrel ft/sec” and “16.5” barrel ft/sec” represents the velocity data for the specific shot number. “AVG 28″ ft/sec” and “AVG 16.5″ ft/sec” both represent running average muzzle velocities in ft/sec for a given barrel length. “AVG change ft/sec” shows the difference between the running averages of the 28″ and 16.5″ barrels. “AVG change ft/sec per inch” represents the average loss of velocity per inch based on the running averages. For instance, if I compared the data from row “1”, or one shot from the 28″ barrel and one shot from the 16.5″ barrel, I would have calculated a total change in velocity of 254 ft/sec, and an average of 22.1 ft/sec per inch. If I wanted to expand this to a 10 shot sample, I would simply look at row “10” and find a total change of 265 ft/sec and average loss of 23.0 ft/sec per inch of barrel. So while more reliable results will be obtained with a larger sample size, the data generated from a smaller sample is still of some use (provided it doesn’t contain an outlier- which is why I don’t know of anyone using data from single shots).
|308 Winchester/ 7.62x51mm NATO Comparison of velocity data
|Shot||28″ barrel ft/sec||AVG 28″ barrel ft/sec||16.5″ barrel ft/sec||AVG 16.5″ barrel ft/sec||AVG change ft/sec||AVG change ft/sec per inch|
Did you shoot any groups?
No, I did not. I did in the 223 rifle and 300 Win Mag posts, and was shocked with the performance of a saw-cut crown. Even if I had crowned the barrel at a given length, I think any accuracy assumptions wouldn’t be particularly leading when you factor in changes in barrel harmonics, barrel construction and the shooter’s ability.
Why didn’t you crown the barrel?
Time. My lathe is a two hour round trip to the range. Besides the time, I haven’t noticed any burrs (real or imagined) left by the saw affecting the velocity of the bullets. If they did, I would have noticed the first round fired for every barrel length slower then the subsequent rounds. This is not shown in the data, nor has was it shown in data for the 223 rifle , 308 Win, 7mm Remington Magnum and 300 Win Mag posts.
To view similar experiments with other cartridges, please see Rifleshooter.com’s barrel length and velocity pages.
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