Modern Historical Intermediate Calibers 010: The 6mm SAW

On the right are the two major iterations of the 6mm SAW, the 45mm steel cased version, and the 50mm aluminum cased version. In the middle is a modified .25 Winchester experimental round used for ballistic testing in the early part of the SAW program. On the far left is 5.56mm M855, which became the eventual chambering for the resulting M249 SAW.

In this installment, we’ll be looking at a very unique round. The 6mm SAW was probably the first small arms round ever designed using computer-calculated parametric analysis, and it was also probably the first American rifle round designed from the outset for steel cartridge cases. The initial design parameters were for a round effective to 1,000 meters, and which accepted a tracer that would give a visible daylight burn out to 800 meters, and this resulted in a caliber of 6mm with a 105 grain slender steel-jacketed lead-cored bullet possessing a high length/drag ratio. Although fired at modest velocity, the sleek bullet retained its energy extremely well, giving the round good long-range penetrating power, especially compared to the 5.56mm rounds at the time. Ultimately, the 6mm SAW succeeded in its ballistic goals, but was rejected on logistical grounds, as the idea of fielding three different rounds (5.56mm, 6mm, and 7.62mm) was not considered feasible.

On to the ballistics:

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The 6mm SAW produced two major variations: A version with a 45mm long steel case, and another with a 50mm long aluminum case. Weights of these rounds were 15 grams for the former, and 11.6 grams for the latter.

Note: All ballistic calculations are done with JBM’s Trajectory calculator, using the ballistic coefficient appropriate to the projectile being modeled, and assuming an AR-15 as a firing platform. Also, keep in mind that there is no single true velocity for a given round; velocity can vary due to a large number of factors, including ambient temperature and chamber dimensions. Instead, I try to use nominal velocity figures that are representative of the capability of the round in question.

EDIT: Since I’ve decided to expand this series to calibers that – while they may still be ballistically relevant – are no longer actually being produced or tested, I have decided to re-name this post (and several subsequent ones) to “Historical Intermediate Calibers”. However, I will retain the same numerical sequence and the word “Modern” struck through. Enjoy!

Nathaniel F

Nathaniel is a history enthusiast and firearms hobbyist whose primary interest lies in military small arms technological developments beginning with the smokeless powder era. In addition to contributing to The Firearm Blog, he runs 196,800 Revolutions Per Minute, a blog devoted to modern small arms design and theory. He is also the author of the original web serial Heartblood, which is being updated and edited regularly. He can be reached via email at


  • Christian Hedegaard-Schou

    Curious why they decided to go with a steel jacket over a traditional copper jacket.

    That could not have been good for barrel life, especially considering the full-auto intention of the round.

    • Kivaari

      Possibly to save on “strategic materials”. Brass being important for other uses. It is a wash on cost to produce. Steel is cheaper, but it wears out the machinery faster, so it needs replacing more often.

    • It was all about the long-range penetration.

      • Kivaari

        The bullet had a conventional copper jacket didn’t it? The case I understand was steel as a strategic materials preservation choice. Neither would have much to do with long range penetration, would they?

        • 6mm SAW had a steel jacket, and that was chosen to improve long-range penetration. In fact, partway through development it was thickened to further help improve penetration.

          • Kivaari

            I wonder if that led to lesser performance in tissue or BG, because the bullet remained intact. Unlike others that break apart? I can see using steel. If you are looking for longer range performance, and penetration of barriers, than it makes sense.

          • I don’t know, honestly. It seems to have been quite the capable penetrator, at least.

          • Kivaari

            Well, hitting and making a hole is a big deal. That’s why I like the 5.56mm it is easier to hit with that some of its competition. And holes in targets, even without the bullet tumbling and going to pieces make the target sick.

          • Stan Darsh

            Were the 6MM SAW bullets at least copper washed?

          • They were gilding metal plated.

  • Amplified Heat

    How many of these are you doing? I haven’t even heard of half these chamberings, and this latest batch appears to be substantially longer/larger than the 5.56. Since geometry is a continuum, what exactly were the ‘bounds’ limiting the cartridges examined for the articles?

    • 6.5x55Swedish

      I think you will see all ronds that fits a military application and is larger than 5.56 NATO and smaller than 7.62 NATO.

      • ostiariusalpha

        6x35mm KAC is smaller than the 5.56x45mm round, but it is still an intermediate cartridge.

    • Originally I was only going to cover the six most commercially and militarily relevant AR-15 calibers, plus 7.62×39. Those became the first seven that I published. However, I received several messages from people saying how much they like the series and asking whether I would be doing more, so I decided to continue making them.

      I haven’t really been using “bounds”, but I have been selecting the rounds based on whether they were designed for rifles or carbines, and their general ballistic characteristics.

      • mig1nc

        This is a great series. Keep it up!

  • mig1nc

    That round is very slender. Were there any concerns about terminal ballistics? As I understand it, that’s one of the reasons Murray advocates for a 7mm over 6.5mm round as the optimum universal military cartridge.

    • Yes, Murray rejects calibers below 6.8mm because he believes they cannot provide adequate terminal effectiveness. However, I don’t believe he is correct. His reasoning is based on scattered historical accounts, many of which involved the use of archaic round nosed bullets that gave universally poor terminal effectiveness regardless of caliber.

      My reading of the research does not in any way suggest that wounds from calibers just below Murray’s threshold are any less severe than those just above it. He and those who agree with him like D.D.S. Roberts use this theory to justify rejecting the 6mm and 6.5mm calibers, but I think this is more an expression of their personal preferences than it is something tangible.

      • mig1nc

        I’ve not read that Roberts rejects 6.5. In fact I’ve read several posts from here where he acknowledges that 6.5 Grendel and similar rounds are superior to 5.56mm in terminal effectiveness. Although he could have changed his mind since then…

        Still though, I’d be curious how this one stacks up.

        • I don’t know that he has the seething hatred for the 6.5mm that Murray evidently does, no, but I’ve seen him make similar arguments.

          Roberts is pretty inconsistent on a lot of things; for a good example take a look at this paper:

          There’s some weirdness there. In the section on small PDW calibers like the 5.7mm, he talks up the 5.56mm, citing people who say they are very happy with the 5.56mm, saying that law enforcement agencies should choose 5.56mm SBRs, etc.

          However, in the section after next, he starts taking a crap on the 5.56mm, saying things like:

          “Obviously, an experienced outdoorsmen would not choose a 5.56 mm / .223 caliber weapon when confronting a dangerous
          animal, yet we ask our LE officers and military personnel to face foes of similar size who pose an even greater threat, armed with a rifle caliber considered too anemic to reliably stop a similar sized animal assailant.”

          It’s not that these two sections directly contradict each other, but their tones regarding the 5.56mm are completely different. One sounds like “choose a reliable caliber like 5.56mm”, and the other sounds like “oh man, 5.56mm is a mess, you have to carefully select the load you run or else it will be mission failure!” This tone seems to oscillate depending on what he wants you to think about the subject round.

          • mig1nc

            I see your point. By the way, thanks for taking the time to respond. I appreciate the dialog.

      • Kivaari

        The biggest failing of all the archaic rounds, was the round nose bullets. The nations that clung to them, long after spitzers were shown to be better both in the air and in the target is baffling. The cost to improve the performance really came down to re-calibrating the rea rsights of the guns. Once done the cost of ammo would come down simply through the use of less material. No more 160 gr 6.5mm or 175 gr. 7mm round nosed bullets but properly designed spitzers. The great reviews of the 7x57mm during the South African wars was based on the 175 gr. FMJ-RN v. .303 RN. ZNeither were great for wounding, just that flatter shooting 7mm out performed the slow .303. Russia and Japan both saw this effect in the war of 1905. I contend a good bullet of 5.5mm to 7mm is going to do its job if driven fast enough. You can just make a better intermediate round by keeping the bore size small. Going up to get the “ideal” bore diameter is a chase that leads to a rifle that can’t deliver the benefits of the 5.5mm rifles. Pretty soon, it is no longer intermediate.

    • Way back in the 1950s, the Ordnance Corps guys determined that, other things being basically equal (SD, LxW ratio, velocity, etc.), smaller bore bullets yaw in tissue *faster*, as a matter of physics.

  • Ben Loong

    I was always curious about this cartridge ever since I read a series of articles about how the SAW competition went down.

    In particular, I always wondered if it would be a viable replacement for 7.62 NATO in the GPMG and DMR role.

    • Stan Darsh

      I’ve always had the same sentiment. The 6x50mm variant should have replaced 7.62 NATO even if they necked it up to 6.5 and utilized a Grendel-esque bullet design or made the bullet construction mirror that of M855.

  • Julio

    “Ultimately, the 6mm SAW succeeded in its ballistic goals, but was rejected on logistical grounds, as the idea of fielding three different rounds (5.56mm, 6mm, and 7.62mm) was not considered feasible.”

    In that case, i.e. if there was never any intention to field 3, or drop 1, one has to wonder why they even bothered to draw up the specification, let alone go to all the time and expense of developing the cartridge.

    Also, “unique” should not have a qualifier. Either things are “unique” or they aren’t.

    • Do you know of another American steel-cased 6mm round with a 10.5mm case base and a 9mm long case neck that is loaded with a 105gr steel-jacketed FMJ?

      Because I don’t.

      The qualifier is there so some grammar nazi doesn’t come in here and yell at me for calling a mass produced round “unique”. 🙂

  • grintch11

    Can we add 5.56 & 7.62 NATO as reference lines to the ballistics charts? As they are the defacto standards for intermediate and full power military rounds.

    With nothing to compare them to, it’s hard to evaluate a rounds performance in the current charts.

    • I am going to write a larger wrap-up post, but given the extent to which I’ve decided to expand this series, I might go back and edit in some baselines, yes.

      • Kivaari

        Good. At least do 5.56mm.