MK318 Mod 0 - accuracy evaluation and chronograph data

[Molon]

Federal T556TNB1 (MK318 Mod 0)






A version of MK318 Mod 0 that is now available to the general public is being sold as Federal "white box" T556TNB1.The SOST projectile loaded in the MK318 Mod 0 cartridge has a nominal weight of 62 grains and a nominal length of 0.87”. It is constructed with a copper base and a small, non-bonded lead core in the ogive section of the bullet. Due to this higher copper to lead ratio, the MK318 Mod 0 projectile is longer than a traditional copper jacketed/lead core projectile of the same weight, but is still slightly shorter than an M855 projectile.










While it is often stated that the SOST projectile used in MK318 Mod 0 is similar to the Federal Trophy Bonded Bear Claw, the form of the SOST projectile more closely resembles that of Federal’s Trophy Bonded Tip projectile (without the tip of course.) As previously mentioned, the MK318 projectile does not have a bonded core.

















The loaded MK318 Mod 0 cartridge has a nominal OAL of 2.20". The projectile does not have a cannelure per se, but the case mouth is crimped into the top relief band. The cartridge is sealed at the case mouth, however not with the asphalt sealant typically found on military ammunition. The primers are sealed and crimped. The casehead is stamped "FC 10". The cartridge is charged with ball powder.








A typical copper jacketed/lead core FMJ bullet will have a specific gravity of approximately 10.2. Due to its higher copper to lead ratio construction, the MK318 Mod 0 projectile has a lower specific gravity. When fired from typical AR-15 barrel lengths with a 1:9” twist rate, MK318 Mod 0 will have a gyroscopic stability factor of approximately 1.3.






When fired from typical length barrels with a 1:7" twist rate, MK318 Mod O will have a gyroscopic stability factor of approximately 2.2.








MK318 Mod 0 Chronograph Data

Chronographing of the Mk318 Mod 0 ammunition was conducted using an Oehler 35-P chronograph with “proof screen” technology. All velocities listed below are muzzle velocities as calculated from the instrumental velocities using Oehler’s Ballistic Explorer software program. All strings of fire consisted of 10 rounds each.












Each round was single-loaded and cycled into the chamber from a magazine fitted with a single-load follower. The bolt locked-back after each shot allowing the chamber to cool in between each shot. This technique was used to mitigate the possible influence of “chamber-soak” on velocity data. Each new shot was fired in a consistent manner after hitting the bolt release. Atmospheric conditions were monitored and recorded using a Kestrel 4000 Pocket Weather Tracker.






Atmospheric conditions:

Temperature: 75 degrees F.
Humidity: 47%.
Barometric pressure: 29.97 inches of Hg
Elevation: 950 feet above sea level




Two different barrel lengths were used in obtaining velocity data; a 14.5” Colt M4A1 barrel and a 20” Colt M16A2 barrel. Both barrels have NATO chambers, are chrome lined and have 1:7” twist rates. Both barrels have low round-counts on them.


M4A1 barrel.




M16A2 barrel.





For comparison, the MK318 Mod 0 ammunition was fired in sequence with two different brands of 62 grain M855 ammunition. The firing order for both barrels was as follows:

1. A 10-shot string of Winchester Ranger M855
2. A 10-shot string of MK318 Mod 0
3. A 10-shot string of IMI M855

Finally, the data:










The Crane Warfare Centers' publication, “U.S. Navy Small Arms Ammunition Advancements” reported that MK318 was “optimized” for the MK 16 with a 14 inch barrel and claimed it has a velocity of 2925 fps at 15 feet from the muzzle (presumably from said 14 inch barrel.) The lot of Federal T556TNB1 (MK318 Mod 0) that I chronographed from the Colt 14.5” M4A1 barrel would have a velocity of approximately 2889 fps at 15 feet from the muzzle (at standard atmospheric conditions.)




accuracy results in post #8 below . . .

[jmart]

For your handload, did you pull from factory cartridges or were you able to source some bullets as components? What powder, charge weight and velocity? Did you do any ladder testing? What OAL did you load these to? Looks like they are loaded longer than factory.

[littlejerry]

Molon,

Thanks for posting this. I'm anxious to see the factory load with a 1/7 5.56 barrel(along with chrono data for various lengths...)

[Ned Christiansen]

Thank Mo, been very curious about this stuff.

[Molon]

Chronograph data added to first post.

[littlejerry]

Thanks again for the update Molon.

I was actually expecting slightly higher velocity in the 14.5" barrel. Wasn't part of the SOST improvements its supposed optimization for 14" carbines? Was this a reference to projectile performance at carbine velocities or was it inferring higher velocities from carbine barrels(I had assumed higher velocities from carbine barrels)?

[Molon]

Wasn't part of the SOST improvements its supposed optimization for 14" carbines? Was this a reference to projectile performance at carbine velocities or was it inferring higher velocities from carbine barrels(I had assumed higher velocities from carbine barrels)?

The Crane Warfare Centers' publication, “U.S. Navy Small Arms Ammunition Advancements” reported that MK318 was “optimized” for the MK 16 with a 14 inch barrel and claimed it has a velocity of 2925 fps at 15 feet from the muzzle (presumably from said 14 inch barrel.) The lot of Federal T556TNB1 (MK318 Mod 0) that I chronographed from the Colt 14.5” M4A1 barrel would have a velocity of approximately 2889 fps at 15 feet from the muzzle (at standard atmospheric conditions.)


.....

[Molon]

Accuracy Evaluation of MK318 Mod 0

An accuracy (technically, precision) evaluation of the MK318 Mod 0 ammunition was performed following my usual protocol. This 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).

The shooting set-up will be described in detail below. As many of the significant variables as was practicable were controlled for. Also, a "control group" was fired from each barrel used in the evaluation using match-grade, hand-loaded ammunition; in order to demonstrate the capability of the barrel. Pictures of shot-groups are posted for documentation.

All shooting was conducted from a concrete bench-rest from a distance of 100 yards (confirmed with a laser rangefinder.) The barrels used in the evaluation were free-floated. The free-float handguards of the rifles rested in a Sinclair Windage Benchrest, while the stock of the rifles 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. A mirage shade was attached to the objective-bell of the scope. 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.




In order to establish a working baseline for the intrinsic accuracy of the 62 grain SOST projectile itself, when fired from a semi-automatic AR-15, I worked-up a SAAMI pressure hand-load with pulled MK318 bullets and fired a 10-shot group of that load from a Krieger barreled AR-15 from a distance of 100 yards. The Krieger barrel has a 1:7.7” twist rate.




Prior to firing the 62 grain SOST hand-load, I fired a 10-shot control group consisting of hand-loaded 62 grain Berger hollow points. The extreme spread for the control group measured 0.66”.






The extreme spread of the 10-shot group of the 62 grain SOST hand-load measured 1.9”.












Since MK318 Mod 0 is intended for use as a combat round, I used AR-15s with chrome-lined, NATO chambered barrels for this accuracy evaluation, as it most likely that these are the types of barrels that this ammunition will most commonly be fired from. It is possible to obtain better accuracy from mil-spec/NATO pressure loads by firing them from an AR-15 that has a stainless steel match-grade barrel with a hybrid chamber such as the Noveske NMmod0 chamber or the Wylde chamber for examples; but you're not going to make a silk purse out of a sow’s ear.

As previously mentioned, it is reported that MK318 was “optimized” for a 14 inch barrel, so it seemed only fitting to evaluate MK318 from a similar length barrel. The first test vehicle used in this accuracy evaluation was a 14.5” Colt M4A1 barrel. The barrel was free-floated with a Daniel Defense Omega rail. (I was not able to use the mirage-shade with this barrel, due to the original standard front sight base on it.) A previous accuracy evaluation of this M4A1 barrel demonstrated that this barrel is capable of excellent accuracy for a chrome-lined, NATO chambered barrel. That evaluation can be viewed here:

https://www.m4carbine.net/showthread.php?t=59364








A control group fired from the M4A1 barrel using hand-loaded 62 grain Berger hollow-point projectiles had an extreme spread of 1.13”.





Three 10-shot groups of the MK318 were fired from the Colt M4A1 barrel from the previously described bench-rest set-up. The extreme spreads of those groups measured:

2.91”
3.22”
2.70”

for a 10-shot group average extreme spread of 2.94”. The three 10-shot groups were over-layed on each other using RSI Shooting Lab to form a 30-shot composite group. The mean radius of this composite group was 1.02”.


The smallest 10-shot group of MK318 fired from the Colt M4A1 barrel is shown below.






The next test vehicle was a 16” Colt HBAR with a 1:9” twist. This is the same barrel found on the Colt 6721 Tactical Carbine. This barrel is one of the most accurate “out of the box” chrome-lined, NATO chambered barrels that I’ve evaluated. The barrel was free-floated with a LaRue Tactical handguard.






A 10-shot group from this barrel fired using hand-loaded Sierra 52 grain MatchKings had an extreme spread of 0.98”.





Three 10-shot groups of the MK318 fired from the Colt 6721 barrel produced the following extreme spreads:

2.98”
2.85”
2.89”

for a 10-shot group average extreme spread of 2.91”. These three groups were also over-layed on each other to produce a 30-shot composite group with a mean radius of 0.82”.


The smallest 10-shot group of MK318 fired from the Colt 6721 barrel . . .






The third barrel used to evaluate the accuracy of MK318 was a 20” Colt HBAR with a 1:7” twist, chrome-lining and a NATO chamber. The barrel is free-floated with a PRI handguard.





A 10-shot group from this barrel fired using hand-loaded 55 grain Sierra BlitzKings had an extreme spread of 1.18”.





Three 10-shot groups of the MK318 fired from the 20” HBAR had extreme spreads of:

2.70”
2.49”
3.24”

for a 10-shot group average extreme spread of 2.81”. As before, the three 10-shot groups were over-layed on each other to form a 30-shot composite group that produced a mean radius of 0.91”.


The smallest 10-shot group of MK318 fired from the 20” HBAR . . .





A summary of the results from this evaluation are shown below.





The table below compares the accuracy of MK318 Mod 0 with several other "mil-spec" loads.





....




Lastly, for the Internet Commandos in our viewing audience, here’s a pic of a sub-MOA group of the MK318 fired from the 16” Colt HBAR from 100 yards; a cherry-picked 3-shot group that is.





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[Molon]

- V e l o c i t y - C o m p a r i s o n -





Following the same chronographing procedures outlined in the first post of this thread, I compared the velocities of T556TNB1 “white box” MK318, to that of AB49 “brown box” MK318. The two versions were fired “back to back” from three different barrel lengths:

 14.5” Colt M4A1 (the same barrel used for the first post)

 16” Colt light-weight barrel, 6520

 20” Colt M16A2 (a different barrel than the one used for the first post).


All three barrels are chrome-lined and have NATO chambers and 1:7” twist rates. The velocities listed below are muzzle velocities as calculated from the instrumental velocities using Oehler’s Ballistic Explorer.


Atmospheric conditions.

Temperature: 72 degrees F
Humidity: 68.1%
Barometric pressure: 30.02 inches of Hg
Elevation: 950 feet above sea level







….


- Accuracy - C o m p a r i s o n -





Using the same 20” Colt HBAR (described in the original accuracy evaluation in this thread) as the test vehicle, I performed an accuracy evaluation of the AB49 “brown box” MK318 ammunition. The evaluation was conducted from a bench-rest at 100 yards in the same manner as previously described.






Prior to firing the AB49 ammunition, a 10-shot control group of hand-loaded, match-grade 55 grain Sierra BlitzKings was fired from 100 yards. That group had an extreme spread of 1.00”.








Three 10-shot groups of the AB49 fired from 100 yards had extreme spreads of:

3.26”
3.42”
3.31”

for a 10-shot group average extreme spread of 3.33”. The mean radius for the 30-shot composite group was 0.98”. The accuracy results are summarized and compared to those of T556TNB1 below.





The smallest 10-shot group of AB49.




.....

[Molon]

The results from accuracy testing of MK318 Mod 0 have been added to post #8 above. For those of you not familiar with the mean radius method of evaluating the dispersion of shot groups, here is some information on the subject.

MEAN RADIUS

Mean radius is the method of measurement of the dispersion of shot-groups used by the US Military for accuracy testing of ammunition. It provides a more useful analysis of the consistency of ammunition and rifle accuracy.

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).”

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 using the group from the above target. 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.