The Colt M4A1 SOCOM Barrel

[Molon]

The Colt M4A1 SOCOM barrel





A genuine Colt M4 SOCOM barrel is 14.5” long and has a medium contour underneath the handguards (not a heavy barrel profile). The barrel has a NATO chamber and a 1:7” twist. The SOCOM barrel has the typical M4 circumferential cut-out located approximately 1.4” forward of the gas block for the attachment of an M203, as well as rectangular shaped cut-outs underneath the handguards on the port and starboard sides of the barrel, located approximately 1.4” aft of the gas block, for the same reason. The SOCOM barrel employs the carbine length gas system.











The barrel stamp located just aft of the flash suppressor reads:

C MP
5.56 NATO 1/7








The barrel has an “F” marked front sight base and a side sling swivel. The “F” mark is located on the port side of the front sight base.











The SOCOM barrel will have a “date code” located immediately aft of the handguard retaining ring. The handguard retaining ring itself will also have cut-outs located at 4:30 and 8:30; again for the attachment of an M203.








The barrel will also have a small “O” stamp, that is located at the chamber section of the barrel, indicating a chrome-lined chamber and bore.







Naturally, the SOCOM barrel will have M4 feedramps and there will be a numeral “4” stamped at the 3 o’clock position on the barrel extension.







Here are a couple pics comparing the SOCOM barrel to the standard M4 barrel.










Accuracy Evaluation



I performed an accuracy (technically precision) evaluation of the Colt SOCOM barrel following my usual protocols. All shooting was conducted from a concrete bench-rest from a distance of 100 yards (confirmed with a laser rangefinder.) For the accuracy evaluation, the barrel was free-floated using a Daniel Defense Omega rail.




The accuracy evaluation used statistically significant shot-group sizes and every single shot in a fired group was included in the measurements. There was absolutely no use of any Group Reduction Techniques (e.g. fliers, target movement, Butterfly Shots).

As many of the significant variables as was practicable were controlled for. The free-float handguard of the carbine rested in a Sinclair Windage Benchrest, while the stock of the carbine rested in a Protektor bunny-ear rear bag. Sighting was accomplished via a Leupold VARI-X III set at 25X magnification and adjusted to be parallax-free at 100 yards. Wind conditions on the shooting range were continuously monitored using a Wind Probe. The set-up was very similar to that pictured below.





The Wind Probe.





Using match-grade hand-loads topped with Sierra 55 grain BlitzKings, I fired three 10-shot groups from the Colt SOCOM barrel in a slow, steady manner from 100 yards. Those groups had extreme spreads of:

0.90”
1.02”
0.93”

for a 10-shot group average extreme spread of 0.95”. All three of the 10-shot groups were over-layed on each other using RSI Shooting Lab to form a 30-shot composite group. The mean radius for the composite group was 0.35”.

The smallest 10-shot group.





The 30-shot composite group.




For those of you not familiar with the mean radius, I’ve included an explanation below.


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[Molon]

A PRIMER ON THE MEAN RADIUS

The Mean radius is a method of measurement of the dispersion of shot-groups that takes into account every shot in the group. It provides a more useful analysis of the consistency of ammunition and firearms (accuracy/precision) than the commonly used method of extreme spread.

The typical method used to measure a group consists of measuring the distance between the centers of the two most outlying shots of a group. This would be the “extreme spread” of the group. We are essentially measuring the distance between the two worst shots of a group. Take a look at the two targets below.









Most people would intuitively conclude that the second target shown is the “better” group. Measuring the two groups using the extreme spread method, we find that both groups measure 2.1”. Once again with the typical method of measuring groups we are measuring the distance between the two worst shots of the group. This method tells us nothing about the other eight shots in the group. So how can we quantitatively show that the second group is better than the first? (Yes, we could score the groups using “X-ring” count, but this does not give us any differential information about all those shots in the X-ring.) This is were the mean radius method comes in. It will give us that extra information we need to better analyze our groups, rifles and ammuntion. If I just reported the measurements of the two groups above using the extreme spread meathod, without a picture, you would assume that the two groups were very much the same. Using the mean radius method shows that the second group is much more consistent. It has a mean radius of 0.43” compared to 0.78” for the first group.

Mean radius as defined in Hatcher's Notebook “is the average distance of all the shots from the center of the group. It is usually about one third the group diameter (extreme spread)” for 10-shot groups.

To obtain the mean radius of a shot group, measure the heights of all shots above an arbitrarily chosen horizontal line. Average these measurements. The result is the height of the center of the group above the chosen line. Then in the same way get the horizontal distance of the center from some vertical line, such as for instance, the left edge of the target. These two measurements will locate the group center.

Now measure the distance of each shot from this center. The average of these measures is the mean radius.

Once you get the hang of measuring groups using the mean radius it becomes very simple to do. While being very simple to do, it is also very time consuming. Modern software programs such as RSI Shooting Lab make determining the mean radius a snap.

The picture below is a screen snapshot from RSI Shooting Lab. The red cross is the center of the group (a little high and right of the aiming point). The long red line shows the two shots forming the extreme spread or group size. The yellow line from the red cross to one of the shots is a radius. Measure all the radii and take the average to obtain the mean radius.









Mean Radius Demonstration

Let’s say you fired a 5-shot group from 100 yards and the resulting target looks like this. (The X-ring measures 1.5” and the 10-ring measures 3.5”.)









The extreme spread of the group measures 2.83”, but we want to find the mean radius (or average group radius.) In order to find the mean radius we must first find the center of the group. By “eye-balling” the target most people would see that the group is centered to the left of the “X-ring” and probably a little high, but we need to find the exact location of the center of the group.

Locating the Center of the Group

The first step in finding the center of the group is to find the lowest shot of the group and draw a horizontal line through the center of that shot.









Next, find the left-most shot of the group and draw a vertical line through the center of that shot.









Now measure the distance from the horizontal line to the other four shots of the group that are above that line. Add those numbers together and divide by the total number of shots in the group (5).









2.50” + 1.03” + 2.01” + 1.30” = 6.84”

Divide by 5 to get 1.37”. This number is the elevation component of the center of the group.

Next we need to find the windage component of the center of the group. From the vertical line, measure the distance to the other four shots of the group that are to the right of the line. Add those numbers together and again divide by the total number of shots in the group (5).









1.76” + 2.54” + 0.45” + 1.19” = 5.94”

Divide by 5 to get 1.19” This is the windage component of the center of the group.

Finding the windage and elevation components of the center of the group is the most difficult part of this process. Once that is done the rest of the process is a piece of cake.

Using the windage and elevation components, locate the position on the target that is 1.37” (elevation component) above the horizontal line and 1.19” (windage component) to the right of the vertical line. This location is the center of the group!









Determining the Mean Radius

Now that we have located the position of the center of the group, the first step in determining the mean radius is to measure the distance from the center of the group to the center of one of the shots. This line is a single “radius”.









Now measure the distance from the center of the group to the center of each of the rest of the shots in the group. Add the measurements of all the radii together and then divide by the total number of shots in the group (5).









0.85” + 1.35” + 1.38” + 0.84” + 1.61” = 6.03”

Divide by 5 to get 1.21”. This is the mean radius (or average group radius) of the group!

Using the mean radius measurement to scribe a circle around the center of the group gives you a graphic representation of the mean radius. This shows the average accuracy of all the shots in the group. This demonstrates why the mean radius is much more useful than the extreme spread in evaluating the accuracy of our rifles and ammunition.









The table below will give you an idea of the relationship between the mean radius and extreme spread for 10-shot groups.








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[rljatl]

Wonder how much smaller the groups would be with 77gr. SMKs?

[rljatl]

Looks like the flash hider on the SOCOM barrel has been pinned on. I've been looking for SOCOM upper. If you don't mind me asking, where did you get the SOCOM barrel?

[GermanSynergy]

Molon,
I always enjoy reading your posts- I feel like I'm in a university classroom.

Thank you for taking the time to conduct these evaluations and share your findings with us here.

[ChicagoTex]

Nice write up and instruction.

Do you happen to have similar data for an original M4 profile barrel, how do the two compare?

I was given to understand that accuracy-wise they should be practically the same, and that the M4A1 SOCOM Profile was adopted because the thicker barrel yields a higher tolerance to extended fully automatic fire.

[pilotguyo540]

Awesome write up! Th*nk you for the education.

[Magic_Salad0892]

Thanks, for this Molon. I love your threads.

This would be awesome to see with an A2 type barrel, or KAC barrel.

You're a great teacher.

[Army Chief]

Nice write up and instruction.

Do you happen to have similar data for an original M4 profile barrel, how do the two compare?

I was given to understand that accuracy-wise they should be practically the same, and that the M4A1 SOCOM Profile was adopted because the thicker barrel yields a higher tolerance to extended fully automatic fire.

Essentially correct -- the higher tolerance in this case has mostly to do with the heat sink characteristics associated with the heavier barrel. They take much longer to heat up, and conversely, also take much longer to cool down after sustained fire.

The accuracy component would likely be more related to harmonics, and it would be indeed be interesting to see a comparison to the baseline non-A1 barrel.

I've got one of the special run SAW 6920HB carbines, which features this same barrel in a non-NFA length (16"). Never really considered whether it was more inherently accurate or not, although there is certainly much to consider in the opening post.

AC

[RogerinTPA]

Excellent evaluation as usual Molon. Well done.

I too am curious about the accuracy of a standard government profile M-4 barrel, using similar or like ammo, compared to the SOCOM barrel.