Modern Intermediate Calibers: Trade-Offs – Introduction

Left to right: .280/30 British, 7.62x39mm M67, 5.56x45m M855, 6.8x43mm SPC XM68GD, 6.5x38mm Grendel 123gr Lapua Scenar, 7.92x33mm Kz.Ptr.43 sME. All of these rounds have different characteristics that affect their ballistic performance and their reliability in automatic firearms. We'll be taking a closer look at these characteristics to better understand the trade-offs in small arms ammunition design.

Left to right: .280/30 British, 7.62x39mm M67, 5.56x45m M855, 6.8x43mm SPC XM68GD, 6.5x38mm Grendel 123gr Lapua Scenar, 7.92x33mm Kz.Ptr.43 sME. All of these rounds have different characteristics that affect their ballistic performance and their reliability in automatic firearms. We'll be taking a closer look at these characteristics to better understand the trade-offs in small arms ammunition design.

Looking at the 24 different calibers we’ve covered as part of the Modern Intermediate Calibers series, some patterns begin to emerge. We see that larger rounds with heavier bullets weigh more, and have more recoil, that more slender bullets shoot further for their weight than other comparable projectiles, and that higher velocity rounds shoot flatter. Each of these patterns corresponds to a trade-off, however, as in some way each “improvement” in performance sacrifices good characteristics elsewhere. Sometimes, these trade-offs are obvious, but sometimes they aren’t.

Moving from real rounds that can actually be found in stores and at auctions to the theoretical and abstract, I want to take a look at what some of these trade-offs are, and how they affect the design of small arms ammunition today. Each post in the series will be brief, giving a short explanation of the trade-off being explored, as well as a corresponding analysis.

7.62x39 features 2

This image shows some of the characteristics of 7.62×39 that aid the reliable function of small arms ammunition. However, each one of these characteristics involves a trade-off that may negatively affect the ammunition or the rifle firing it in some way. How these trade-offs are balanced is an important part of small arms ammunition design.

 

There are some things we won’t be talking about in this series, such as propellant and primer selection, crimp type and strength, etc. These things are important, of course, but for the purposes of this article we will only be considering trade-offs in the basic characteristics of a type of ammunition, such as projectile weight, case length, capacity, taper etc.

The goal of this series is to help our readers reach a point where they have a working vocabulary of the fundamentals of small arms ammunition design, and understand the trade-offs created by changing one aspect of the ammunition’s design, or another.

One unfortunate limitation of this series is that it only will deal with classic metallic-cased centerfire ammunition, with a focus on intermediate rounds. Some of the principles covered here therefore will not be applicable to next-generation polymer cased or caseless ammunition, as the trade-offs for those kinds of ammunition will likely be different!

Our first stop in exploring the trade-offs inherent to ammunition design? Stay tuned, as we talk about caliber selection.



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 can be reached via email at nathaniel.f@staff.thefirearmblog.com.


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  • Trey Heldmann

    I am extremely excited for this series on trade offs. I really enjoy all of your articles. I love that they are 95% raw facts/data, makes for a very informative read for those like me that have a strong affinity for firearms, but lack the expert knowledge that you have attained. As for your opinions, I respect them as well, even if I may not always agree with you, as they are always well reasoned.
    I guess I am just trying to say thank you for passing your knowledge of all things firearms to plebs like me. ☺

  • somethingclever

    Love this series. Keep up the good work.

  • Wolfgar

    This is a needed but sometimes overlooked aspect of a military cartridge especially by armchair experts such as myself. Looking forward to this series.

  • Vitor Roma

    The one I really dislike is the 6.8mm. Heavier, more recoil and less mag capacity than the 5.56 for bullet shape with mediocre BC. The 6.5 has those trade offs, but at least gives an amazing BC.

    I know some folks will say “the 6.8 has great short to medium range perfomance”. But bragging about seems rather silly, since the 5.56 does absolutely great in this same in the form of M855A1 or 70/77gr match bullets that also does better in the medium to long range.

    • yodamiles

      Have anyone try to neckdown 6.5 grendel to fit 5.56 bullet? Loading it up with high bc 77 gr. or 80 gr 5.56 seems really good with Grendel case since you don’t have to worry about overall-length and case capacity.

      • ostiariusalpha

        Most wildcats go with 6mm bullets, but 90 gr .224 projectiles would be easy-peasy in the Grendel case.

        • yodamiles

          Exactly, it would be slightly lighter and extremely flat shooting round. There’s 20 Grendel (.204 projectile in grendel case) so I don’t know why people haven’t try .224 yet

        • yodamiles

          Ok. After some research, I found out about .224 AR Mag which is basically 6.5 Grendel neckdown to .224.
          They are getting 2900 fps with 82 gr. Berger BT and 2800 fps for 90 gr. (BTW, this is from 28 inches!!!! barrel)
          Sounds really good.

          • Kelly Harbeson

            Seems like the same issues as with 6.5 Grendel only worse. 6.5 Grendel only shows its advantages out of a 24″barrel or longer. A 28″(!) tube is a non-starter for me.

  • billyoblivion

    > We see that larger rounds with heavier bullets weigh more, and have more recoil

    OMG, that’s BRILLIANT. It’s almost like you’ve discovered a law of physics or something.

    • tts

      Oh come on don’t be so harsh.

      Thats common sense to lots of people who shoot but this site gets lots of gun noobs too and if no one mentions this stuff how are they supposed to learn?

      FWIW I’ve found most gun noobs think all the recoil comes from the cartridge powder and don’t make the connection to the bullet weight having a impact until you tell them.

    • valorius

      lol

  • Smedley54

    I admire your courage and your thick skin.

  • valorius

    A .220 swift caliber assault rifle would be epic…shame barrel erosion is so severe in that caliber.

    • ostiariusalpha

      Pretty sure Nate will cover the details of why trade-offs for higher energy and velocity that skirt being overbore are an essential challenge for ammunition designers.

      • valorius

        .220 is a non starter because of barrel erosion issues, at least until some new metallurgical breakthrough is made.

  • valorius

    If 7.62×39 is so great one must wonder why the Russians abandoned it decades ago.

    • CruisingTroll

      Tradeoffs. Consider the extreme, the .50 BMG. Why isn’t every grunt armed with it?

      • valorius

        Because the M107 weighs over 35lbs loaded.

        • CruisingTroll

          That’s ONE of the tradeoffs. On the other hand, you get a round that will go through ANY body armor or helmet, as well as 2 or 3 enemy soldiers. The .50 BMG is a very, very lethal, very long range round. Why, it’s perfect… 😉

          • valorius

            You’re living up to your screen name, that’s for sure.

          • CruisingTroll

            Thank you, thank you. It’s a gift.

            Seriously, what I’m trying to get to via the Socratic method is that folks consider what are the tradeoffs that the Soviets/Russians made when they abandoned the 7.62×39 for the 5.45xwhatever? Keep in mind that a key consideration is this is for INFANTRY usage. Not hunting. Not personal defense. Not long range target shooting or sniping. Infantry. They saw some advantages to the trade. Be sure to look at it from the Russian perspective, not a Western perspective. If you’re patient, it’s likely that our good author will make it all clear. (The fact that he hasn’t shown the downsides of the x39 yet gives one hope.)

          • valorius

            I agree with the Russians switch to 5.45mm. It is a superior caliber for virtually all military purposes.

            And i said it without hyperbole 😀

          • The Brigadier

            OMG here is someone who makes the statement that so many both agree and disagree with – that .556 is the all around all purpose cartridge. Its doesn’t work for cleaning products and it doesn’t work for rifle cartridges either.

  • Kelly Harbeson

    Nathaniel F, weren’t you going to publish a part 2 of your research on the 5.45×39 cartridge?

  • Mike Carey

    Glad to see this Series. This is exactly what I’d like to explore and learn more about!

  • d s

    I love my Savage 17 WSM. 4.5-14×44 Scope. Mean little round.

  • The Brigadier

    Geez, Nate started out talking about the AK round and everyone is weighing in about all the cartridges. Let’s wait for the rest and then start the arguing. I can’t wait for his series on battle rifle cartridges.