223 Remington/5.56mm NATO Barrel length versus Velocity- Short Barrels- 6 to 14 inches

In 223 Remington/5.56 NATO, velocity versus barrel length: A man, his chop box and his friend’s rifle, we cut the barrel of a factory Remington 700 chambered in 223 Remington back one inch at a time and recorded the average velocity for four different 223 Remington and 5.56mm NATO cartridges.  The data set generated from that post provided imperial values for muzzle velocities from 26″ to 16.5″.  A few readers suggested mounting the barrel in a pistol and continuing the test for shorter barrels- we liked the idea.  In this experiment, we gathered data using the same barrel from the first 223 Remington/5.56mm NATO experiment (on a pistol action), with the same four kinds of ammunition from 14″ to 6″.

Building a bolt-action pistol for the test

As most shooters know, nuances of gun laws can become quite complex, especially when looking at how to build a pistol.  For instance, it is OK to turn a new receiver into a pistol, or a pistol into a rifle, however, turning a rifle into a pistol is not allowed per the National Firearms Act (NFA).  Since simply configuring the Model 700 rifle used in the first post as a pistol is not allowed, another method was required.  After speaking to the legal team at Rifleshooter.com, we arrived at a solution, build a bolt action pistol from a virgin receiver.  As long as the receiver was new, never a rifle, and the finished pistol conformed to NFA requirements, the test could proceed.

To build the bolt-action pistol used in this part of the experiment, I ended up mounting the same barrel that was removed from the rifle in the first post onto a virgin Remington Model Seven receiver.  The Model Seven is a scaled down version of the Remington 700 and uses the same barrel tenon as the Model 700.  Fortunately, when I installed the barrel (cut to 14″) and lug on the receiver, the headspace (1.4706″+) and bolt nose recess worked out (interchangeable parts rule!).  For a stock, I contacted Modular Driven Technologies and ordered one of their LSS chassis systems.  The legal team told me to remove the threaded rear of the chassis so it “wouldn’t readily accept a stock” (their words, not mine), so I cut it off.  A Timney trigger, Nightforce 20 MOA base, Harris bipod and Trijicon RMR finished off this one of a kind pistol.

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+.

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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Ballistic data was gathered using a Magnetospeed barrel mounted ballistic chronograph.  At each barrel length, the pistol was fired from a bipod, with five rounds of the UMC 223 Remington 55-grain FMJ, Federal XM193 55-grain FMJ, and Winchester M855 62-grain FMJ.  Since I didn’t have a lot of Black Hills 68-grain heavy match ammunition available from the same lot, I only fired 3 shot groups at 14, 12, 10, 8, and 6 inch barrel lengths.  Average velocity and standard deviation was logged for each round.  Since I would be gathering data on 32 different barrel length and ammunition combinations and would not be crowning the barrel after each cut; we decided to eliminate gathering data on group sizes.

Once data was gathered for each cartridge at a given barrel length, the rifle was cleared and the bolt was removed.  The barrel was cut off using a cold saw.  The test protocol was repeated for the next length. Temperature was 55F (the temperature was 45.7F when we conducted the rifle portion of the experiment).

I stopped at 6″ because I was unable to attach the MagnetoSpeed barrel mounted ballistic chronograph to such a short barrel length.  Anticipating this, I brought an Oehler 35P proof chronograph with me to the range, however, it was unable to detect the bullets at the lower velocities.

Results:

So what did we learn?

1. As expected, the rate of change in muzzle velocity for all four cartridges was significantly more with a barrel length below 14″ when compared to decreases observed as the rifle barrel was cut.
2. From 14″ to 6″, average velocity loss was 106 feet/sec per inch for the UMC 55-grain, 104.6 feet/sec per inch for the Federal XM-193, 87.5 feet/sec per inch for the Winchester M855, and 82.4 feet/sec per inch for the Black Hills 68-grain heavy match.  The rifle data from 26″ to 16.5 was 22.5  ft/sec per inch for the UMC 223 55-grain cartridge, 25.7 ft/sec per inch for the Federal M-193 cartridge, 30.3 ft/sec per inch for the Winchester M855 cartridge, and 22.8 ft/sec per inch for the Black Hills 68-grain heavy match cartridge.
3. As the barrel length decreased, the muzzle velocity decreased at a great rate, see tables above.

Closing

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 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) 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 Rifleshooter.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
1	2835	2835	2581	2581	254	22.1
2	2814	2825	2533	2557	268	23.3
3	2821	2823	2541	2552	272	23.6
4	2823	2823	2551	2552	272	23.6
5	2824	2823	2601	2561	262	22.8
6	2834	2825	2572	2563	262	22.8
7	2811	2823	2587	2570	252	21.9
8	2816	2822	2546	2564	258	22.5
9	2821	2822	2545	2562	260	22.6
10	2827	2823	2520	2558	265	23.0
11	2835	2824	2584	2560	264	22.9
12	2820	2823	2592	2563	261	22.7
13	2825	2824	2554	2562	261	22.7
14	2820	2823	2551	2561	262	22.8
15	2842	2825	2585	2563	262	22.8
16	2833	2825	2573	2564	262	22.7
17	2825	2825	2540	2562	263	22.9
18	2813	2824	2492	2558	266	23.1
19	2791	2823	2550	2558	265	23.0
20	2797	2821	2546	2557	264	23.0
21	2836	2822	2567	2558	264	23.0
22	2850	2823	2541	2557	266	23.2
23	2826	2823	2559	2557	266	23.2
24	2842	2824	2478	2554	271	23.5
25	2838	2825	2537	2553	272	23.6
26	2831	2825	2569	2554	271	23.6
27	2842	2826	2601	2555	270	23.5
28	2833	2826	2534	2555	271	23.6
29	2796	2825	2578	2555	269	23.4
30	2810	2824	2536	2555	270	23.4

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.

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 used 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.

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.

What’s next?

I’ll be combining my pistol and rifle length data sets and presenting a more comprehensive report of my findings. Keep in mind that I fully understand some of the limitation of doing this, including; the headspace measurement is different by .003″ between the two platforms, ambient temperatures were different at both tests (47.5F versus 55F), the barrel has a 223 Chamber and a 1:12 twist.  Given these limitations, I still feel the consolidating the complete data set in one place will be valuable.