Testing the 5.56mm MK318 MOD 0 / 5.56 SOST


The 5.56 SOST  is the ammo currently being used by the USMC and other troops in the GWOT,  it has not been available for the public to buy for very long so I, like a lot of other people have been curious about how it performs. The round was designed to  have better terminal performance and accuracy from short barreled rifles with a muzzle velocity of 2925 fps from the 14.5 inch M4 barrel.  The round is a OTM,  this is important to remember as it is not the same as a hollow point.  It does however, offer up better performance and is barrier blind. The bullet itself has a thick copper base and a lead front end with the usual open tip that results from the process of making the bullet.  Also present are the rings seen around the bullet much like a barnes triple shock X.  A lot of people assumed and still do that the SOST is a barnes solid copper TSX, but it is not.

After getting my hands on some of the ammo I decided to test it for accuracy first. Since the ammo is intended for combat weapons, I chose to use my Colt 6940  with milspec barrel with 1/7 twist. I did put a Leupold 18x target scope on the carbine though so i could get all the accuracy out of it I could.  I fired 2 strings of 5 shots at 1 inch dots and one string of 10 shots of M855 to compare it to the common military load all at 100 yards. It was around 1100 AM , sunny with a  2 oclock 18 MPH wind.

Other then the normal 1st round flyer you usually get from hand cycling the action as opposed to letting the recoil operate and chamber  the next few rounds, the ammo showed some  great potential. The group on the left  was a little worse due to my own bad trigger work and lost concentration on one shot, but still pretty good considering. Now I know some claim  5 shot groups tell nothing, but if you over lay to  strings you get a pretty good idea  and you keep barrel heat down and mirage off the barrel messing with the scope.

After shooting the groups I decided to test out how the round matched the trajectory of the M855 since that is one of the things the SOST round was meant to have in common with the green tip. Out to 300and 400 yards  the SOST did match the BDC of my TA31F  ACOG and zero of other scopes and Aimpoint RDS I had on hand zeroed for the green tip. Also when comparing the position of the groups in relation to the aiming points, you can see on paper that the zero held very close for both rounds.

Now, the real surprise for me, was how good the lot of M855 I shot turned out to be!  Many gun board expurts and gun magazine expurtitions will gladly tell you how terrible green tip is in the accuracy department along with its many other flaws , makes it  slightly more useful then tits on a boar hog. After getting the carbine hot  after some drills using the SOST and playing around, I fired off 10 rounds fairly quickly  with the Colt/18x combo.  The results left me a little surprised, rarely have I seen green tip shot for accuracy  with sand bags, a table and a target scope further then 50 yards. And I do not recall having seen any pictures of it doing as well in a true 100 distance group.  I shot the green tip with all seriousness and the same concentrated effort I did with the SOST and the results were pleasing and a little surprising to me.  I have never taken the green tip seriously  enough for my own needs in the accuracy department so this will indeed lead to more testing of the M855 if for no other reason  then to see if this was a fluke.

Now I do not mean to sound like I think green tip is crap, I have seen some  good performance out of it at longer ranges on coyote size targets and even man sized targets. But on the other hand, I have seen some terrible accuracy from it too. Of course lots and different MGGs have as much to do with it as anything, not to metion the different shoots and the quality or lack of  in the guns used.

Back to the SOST, I think its a pretty good round from what I can tell. It is not MK 262 or TAP 75 gr. But for general issue to everyone, it is an iprovement in my humble opinion.  I have not shot anything living with it yet, but I will. Also, in the next few weeks I will test it though a few “barriers” like auto glass, wood, and wall paneling. Hopefully I will get to shoot through some auto doors as well.  I would not use it in my own house if over shooting is a issue, but I would use it for anything else at this point if you can find enough for SHTF bulk storing. And you have the benefit of practicing/training with M855 and still being able to shoot the SOST without a zero change. At this point I can not say it is better then any MK 262 top load in accuracy, but it is not meant to be, but, it is better then M193 by a long ways and it shows the ability to shoot a lot better the M855 and even if it just matches it in accuracy you still have the benefits of better terminal performance and barrier penetration while still holding together to hit the target behind if power point presentations can be believed. Hopefully my future testing will offer at least a tiny sample to help prove or disprove the new round.


  1. It’d be interesting to know how that ammo performed out of a 14.5″ barrel on an actual M4…

    The other question, aside from accuracy, is lethality: Just what is that projectile doing inside a human body, and is what it is doing enough to properly put an end to the activities of that body?

    I personally happen to think that the five-and-a-half inches that they chopped off the barrel to make the M4 were inches that they should have done more research on; the 16″ barrels seem to be the sweet spot in regards to both accuracy and lethality, while at 14.5″, a lot of the ammo that does really well in 16″ seems to have really iffy practical results downrange.

    It’s unfortunate that nobody has ever done the requisite research to actually get the hard numbers for this stuff, and that we’re left with doing second-order and third-order proxy research to determine objective truths out of subjective impressions gained on the two-way range. I’m not gonna volunteer to be shot with M855 or M855A1 out of an M4, but at the same time, I’d like a little more confidence that the nice men with ululating voices in the distance that I’m shooting at are gonna stay at the distances I’m shooting them at…

    I’m not at all a fan of this whole “overmatch” BS, but at the same time, I have purely subjective impressions that the 5.56mm in M855 or M855A1 ain’t being all they could be, particularly out of an M4 with a 14.5″ barrel. I still think they should have built the carbine out of a 16″ with a mid-length gas system and a nice mid-weight CHF barrel that wasn’t compromised by legacy fittings for obsolete grenade launchers.

    • I think that was part of the reason behind this and the M855A1. Change in power to get similar velocities out of the 14″ as they would out of the 20″. And I think burn rate was probably in there too. But I’m no expert, so don’t quote me.

      • The thing is, you don’t need gun tearing velocities to get good results at range. One of the best loads out there, the 75 grain gold dot, was loaded to .223 pressures, but still did great things by combining good ballistic coefficient with low velocity threshold for expansion. And on top of that it’s pretty darn accurate.
        G-d, I miss when you could buy it for like $0.60/round. Quality defensive rifle ammo that’s not more than $1/round, imagine that!

  2. Such a neat little round developed in a join effort by USMC together with manufacturers using commercial off the shelf technology. Thank G-d Army scrapped that, paid a billion dollars to develop a not-properly barrier blind bullet then used the senate to ram said bullet down Marines’ throats 🙂

  3. Any ammo is hard to find now, but Mk318 has always been a needle in the haystack. Shawn has some hardcore ninja skills when it comes to obtaining ammo.


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