DSEi 13: ST Kinetics Ultimax 100 Mk5


Ah, the STK Ultimax 100 Mk5. The darling of gun bloggers and youtubers, but alas never given the chance to prove itself in the real world. General Dynamics entered it into the USMC IAR competition but it lost to the HK IAR. ST Kinetics continue to promote it at arms shows so maybe one day it someone somewhere will adopt it.


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Steve Johnson

Founder and Dictator-In-Chief of TFB. A passionate gun owner, a shooting enthusiast and totally tacti-uncool. Favorite first date location: any gun range. Steve can be contacted here.



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  • Big Daddy

    I have read reports from people that have actually used them and they are not very good. Having fun with full auto is one thing. For real combat with an issued and that’s the key word for those who were never in the military, issued, this is not a good weapon. It does not stand up well to the conscript mentally of most soldiers. It is designed to be inexpensive and that’s why the M27 probably won, it’s built to stand up to the rigors of everyday use by the military. Overall it’s a great design, especially with the original drum magazine. I don’t know if this incarnation is any improvement though. The barrel looks long and being un-shrouded would make me nervous to use even with gloves. Conceptually it is better than a H&K M27 but in reality cannot match it. I still think the M27 is a gun looking for a job.

    • Cornelius Carroll

      What shocks me is that there’s no visible fluting or dimpling of the barrel (which the IAR has). You’d think they’d take cooling into account with something that’s meant for sustained fire.

      • Big Daddy

        If I am not mistaken it has a quick change barrel, no need to do that. That also adds more money to the weapon cost which is designed specifically to be inexpensive.

        • Cornelius Carroll

          Fluting/dimpling the barrel really isn’t that hard to do, or expensive. Certainly not more expensive than adding another barrel. IMO, a quick change barrel for a lightweight IAR that’s intended to be “mobile” (read: not a stationary emplacement) seems contradictory to me.

          • Observer

            Well, besides the open bolt, the ROF is not so high as to overheat the gun that fast. This is in addition to the forced “fire breaks” that the mag change will force on the user, so it’s much much more likely that you will run through all your ammo before the barrel overheats. Especially when mag changes force you to take a cooling break.

    • Slim934

      ” It does not stand up well to the conscript mentally of most soldiers. It is designed to be inexpensive and that’s why the M27 probably won, it’s built to stand up to the rigors of everyday use by the military. ”

      So they do not hold up well to field conditions? Is there any real data to show this is the case? I’ve just never seen any real information on the actual usage of these things.

      • Big Daddy

        It’s anecdotal from people in the Singapore military and as the other poster said probably the earlier versions.

        • Observer

          It’s actually also the blanks used in Singapore. Cheap ass blanks with the minimum amount of powder needed to cycle the weapon. Which sometimes isn’t enough. Make that many times. Hell, even my M-4 had problems, the blank would only cycle the bolt carrier half way and even before the blank could be ejected, it would slam forward and crimp the cartridge between the port and the bolt carrier.

          It was even worse with the U-100, the recoil system negates the recoil cycling almost the instant the round fires, which often means that the blank gets crimped or that the bolt carrier doesn’t get far back enough to let the next round load up from the mag (no feeding). This is why many people say that the U-100 doesn’t like blanks. Reliability shoots up much much higher with proper live rounds.

      • pick a name

        ” Is there any real data to show this is the case?”

        It has only been adopted by Singapore and some third world nations despite having been on the market for three decades.

      • kengster

        You would probably have to ask an armourer, as a dumb grunt I never personally encountered or heard of problems with the function of the Ultimax but the peripheral parts were not well thought out. The bipod was easily damaged, the baked on finish on the receivers would scratch easily and expose bare metal which would then promptly rust. The stock on the Mk1-3′s could fall off (fixed in the Mk4). Having said that even old Ultimax’s still fired when you fed them. I think at the core it was a good weapon that was spoiled by poor execution.
        The biggest problem was probably the conscript mentality. Most of the SAF are conscripts serving their national service, the general idea is to do your time and get out fast. This mentality does not bode well for weapons care. It’s fine if you are issued a shiny new weapon but getting an old weapon means inheriting a whole lot of issues. The only weapon that kept going no matter what was the MAG 58′s (M240).

    • kengster

      I used Mk3 and 4′s when I was in the Singapore Armed Forces. Old versions were not well liked because the receivers rusted at a drop of a hat but new Mk4′s were great. Cleaner and far more reliable than the M16S1 that was standard back then, very controllable and responsive. I don’t know why the USMC had to specify open and closed bolt firing on the Mk5, I could fire single shots on full auto no problems at all. Honestly I’d prefer the Ultimax to the old or new rifle.

      • Big Daddy

        I have read about open bolt not being as safe on semi auto which is used in CQB more as a rifle. There’s more to it and many things have been written about it. I have no experience with LMGs, just the M60 GPMG. The Ultimax is the most controllable full auto weapon there is, no recoil, a brilliant design as was the original drum magazine for it. It looks like the Negev took some of it features from certain visual cues.

        • Observer

          It’s not about “safe”, they wanted first round reliability, which means that they wanted the rifle to be able to fire at least one shot before it did funny things. In a closed bolt, the round has been cycled into the chamber and is ready to fire, while open bolts still have to cycle the round in before firing. This adds one more possible “jamming” point which they were not comfortable with. It does not mean the round WILL jam, it means that they were afraid it would. In comparison, the 249 is used in CQB and it’s an open bolt too. Do you hear many people complaining of first round failures often?

          Ironically, I have encountered first round failures in my M-4 too, especially if the charging handle was not pulled back far enough and the bolt carrier didn’t push the round all the way into the chamber.

      • DiverEngrSL17K

        An excellent and entirely relevant observation. The Ultimax 100′s mixed commercial success really has more to do with that eternal bugbear of gun manufacturers — “market timing” — than it does any other set of factors. Combine this with the many political complexities of large-scale military small arms acquisitions and you have a pretty complicated scenario on your hands. An in-depth examination of the Ultimax’s developmental history and the timeline vis-a-vis the original SAWS programme is but one example that will bear this out.

    • Raven

      The M27 is the USMC’s way of getting HK416s, that’s all there is to it. They very clearly wanted 416s because they’re the new thing in AR-pattern guns, but nobody would sign off on letting them buy any as M16A4 replacements. So they came up with the IAR as a way around it.

      • Steve (TFB Editor)

        Yes. It was a way of adopting a new rifle, in this case the HK416 was selected, without out going through a joint taskforce and compromising with the Army, Navy and Airforce. The Army clearly does not want a new carbine, so they were probably happy the USMC went the route they did.

        • Patrick Mingle

          If that is the case are they going to continue to field M249s?

          • Raven

            As I recall, yeah. They’d be taking an enormous step backwards if not.

      • gunsandrockets

        The IAR was an open competition. Are you suggesting the test results were rigged so the HK entry would win?

        • Observer

          If anyone cared to remember the original solicitation, it fitted the U-100 almost perfectly, when the USMC wanted a “non-developmental” “off the shelf” solution. Unfortunately, after 3(?) years of “non-development”, the requirements changed so radically that the U-100 was not even in the running. There was really no competition, the entry bar got shifted so drastically that it could not even enter the start gate.

          Go Google the original solicitation, then compare with the final IAR solicitation. Almost totally different.

          • gunsandrockets

            So the failure of the Ultimax entry in the initial selection phase was due to the changes in IAR requirements? Really?

            The primary difference in those IAR changes you mention was dropping the requirement for a large capacity feed, such as a drum. That didn’t push the Ultimax entry out as you seem to suggest, it just opened the competition to more mag fed entries.

            I like the Ultimax. The problem is the weapon provided to the IAR competition was apparently junk. The sights were so poorly mounted it wasn’t even possible look through them to zero the weapon! That’s not some behind the scenes sneaky act by the USMC, that was a failure by the competitor.

        • Raven

          There were basically four entries that made it to testing: the M27, the FN HAMR, and two Colt designs. LWRC entered one that wasn’t accepted, and General Dynamics tried with the Ultimax, which also went nowhere. The two Colt entries were essentially a 16″ AR with a bigass heat sink under the handguard (DI, closed-bolt), which obviously wasn’t going anywhere. The HAMR was…weird. It was supposed to be able to switch from closed-bolt to open-bolt via a “thermal actuator” in the chamber that I suspect was some kind of bi-metallic switch. It was hamstrung by being based on the generally unwanted SCAR-L/MK. 16 platform, however. So the H&K was really the only one with a chance.

          • gunsandrockets

            Sorry, your recitation of the IAR selection process provided zero evidence in support of your conspiracy theory. USMC did not rig the competition in favor of the H&K bid. Do you really think the competitors of H&K would let the USMC get away with an illegal conspiracy?

            The problem with the conspiracy theory is that it depends on as evidence just the final outcome of the IAR program. The theory completely breaks down when the elaborate process the USMC used to get to the IAR winner is examined.

            Many competitors participated using many different approaches to the USMC requirements for the IAR, but the H&K entry won. Some of the competitors to the H&K entry, that on paper looked superior such as the Ultimax, didn’t even work well enough to make it to the final stages of the competition.

  • Ripley

    6.8 kg with 100 rnd it says on the left. 7.2 kg it says on the right.

  • Dr. Zarkus

    The Mk.2 was used during the Balkan wars in the 90′s. Slovenia & Croatia bought a bunch (along with the SAR 80/88), and were used in Croatia’s war for independence, and in Bosnia. Not sure how they held up.