Have you ever wondered how much barrel length effects muzzle velocity on your rifle? Well I did, so I cut up rifles chambered in 223 Remington, 308 Winchester, 7 mm Remington Magnum and 300 Winchester Magnum to gather empirical data.
The purpose of this post is to combine the barrel length and muzzle velocity data gathered in the Rifleshooter.com’s last two 223 Remington/5.56mm NATO barrel length experiments. To recap how that data was gathered, in 223 Remington/5.56 NATO, velocity versus barrel length: A man, his chop box and his friend’s rifle, I cut a 26″ long factory Remington 700 chambered in 223 Remington back one inch at a time and recorded the average velocity and standard deviation for five shots of four different kinds of ammunition. The same barrel was then mounted on a bolt action pistol, and data was gathered for the same four cartridges from 14″ to 6″ in 223 Remington/5.56mm NATO Barrel length versus Velocity- Short Barrels- 6 to 14 inches.
The rifle used to record data from 26 to 16.5 inches (above).
Review of test protocol
I conducted my first 223 Remington/5.56mm NATO barrel length test 17 months prior to conducting this one. I wanted to control for ammunition and barrel. Using the same barrel, I was able to control for the barrel and chamber. Fortunately, I also had enough left over ammunition from the previous test, so I was able to use the same four kinds of ammunition from the same lot numbers as the 26″ to 16.5″ test.
The four different factory loaded cartridges are; Remington UMC 223 Remington 55-grain U223R3, Federal XM193 5.56 M193 55-grain ball (the box is labeled both XM193 and M193 and has an 02 head stamp),Winchester M855 (08 head stamp) and Black Hills 223 68-grain Heavy Match ammunition.
A quick note on chambers and ammunition, 223 Remington and 5.56 NATO chambers and ammunition are not the same or necessarily interchangeable. 223 Remington is a SAMMI specification and 5.56 NATO is a NATO specification. Without getting overly involved, chamber dimensions are different with 5.56 NATO dimensions being more generous and the 5.56 ammunition running hotter than its 223 counterpart. In our case, our 223 pistol fired 5.56 safely, however, this may not be the case with your particular firearm. Remember, only use ammunition that the manufacturer of your firearm recommends.
The barrel rate of twist is 1:12″. We recognize this is the wrong twist for stabilizing heavier bullets, however, we feel that the velocity information gathered would be worth the effort of firing the heavier bullets. It should be noted, when this barrel was installed on a rifle prior to testing, we were able to shoot some 2 MOA groups with the M855. The 68-grain Black Hills load opened up to approximately 6 MOA.
The pistol’s headspace was measured at 1.4706+ and the rifle’s headspace was measured at 1.1676″.
Velocity ammunition was gathered using a Magnetospeed V3 chronograph. For each barrel length, five rounds were fired of each
The following products were ordered from Brownells for this experiment:
- Magnetospeed V3 chronograph
- Timney trigger
- Accurate Mag 223 AICS style magazine
- Harris BR bipod
- KMW POD-LOC
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Combined barrel length and velocity results (note the data jumps from 16.5″ to 14″ in the tables shown below):
Note the Black Hills 68-grain Heavy Match data shown below has an increased interval (2″) between data points for lengths 14 inches and below. This was due to a limited supply of ammunition.
In the data sets shown above, the increase of velocity loss per inch of barrel at shorter lengths. This is an intuitive finding. For information on how velocity loss effects external ballistics, please see the original two posts.
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. For the second phase of the test, I did bring a Fluke IR thermometer to record barrel temperatures, the highest temperature recorded was 127F, lower than I had anticipated.
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 (or 3 rounds in the case of the Black Hills 68-grain heavy match at lengths 14″ and below) 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 dataRifleshooter.com|
|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?
I did in the original 223 rifle experiment 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 308 Win, 7mm Remington Magnum and 300 Win Mag posts.
Did you notice any pressure signs firing 5.56mm NATO ammunition in a 223 Remington chamber?
No, I did not.
How did you select the ammunition?
I used the same ammunition from the same lot numbers that I used in the rifle length experiments. For a data set using lighter bullets please see Ballistics by the Inch and Accurate Reloading. They both did similar experiments (before I did) and have a lot of good information.
How did your M855 velocity data compare to other sources?
The best study I was able to find on this is “Barrel length studies in 5.56mm NATO weapons” by Dater and Wong. The authors, who are primarily looking at pressure, also recorded muzzle velocities, and use solid methodology in their experiment. While some of my velocities for a given barrel length are higher than those he recorded, the trends both of us observed are similar. The authors do mention their ammunition was stored in a cooler prior to being loaded, I suspect this, along with a different chamber and operating system may account for the lower velocities shown. It is worth noting Dater and Wong also found M855 velocity peaked at 20″, this does not match my experiments either. In Dater and Wong’s experiment, they were mounting a pressure gauge in 1″ intervals to the barrel. This required a drilled and tapped hole, which was plugged when not in use. I suspect that these series of holes had an effect on muzzle velocity, which I hypothesize yielded lower velocities at given lengths and a slow down in muzzle velocity past 20″.
How did the MDT LSS chassis work on a pistol?
It worked like a champ. I used both AICS and Accurate Mag magazines throughout the test. The Accurate Mag magazine worked better. The LSS provided a great interface with the pistol and allowed easy removal of the barreled action when I needed to cut it.
How cool is a bolt-action pistol?
Very. The best part about building a bolt-action pistol on a modified MDT LSS rifle chassis is the detachable magazine. The single shot XP-100 shooters from back in the day must be jealous. I was inspired to build mine after seeing one Cody Weiser of Cody Weiser Firearm Refinishing built on an MDT LSS chassis, I knew I needed to make one. Cody’s website can be found here.
How confident are you in the MagnetoSpeed chronographs results?
Very. Take a look at MagnetoSpeed V3 versus Oehler 35P: Chronograph comparison and review for a detailed analysis if its performance alongside an Oehler 35P.
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