How To Find Case Capacity With SolidWorks

556casevolume

A CAD program is a powerful tool, and can be extremely useful for the amateur handloader and cartridge designer. One of the things that I find most handy in the context of experimenting with hypothetical wildcat cartridges is using SolidWorks or another analytical CAD program to find the internal volume (case capacity) of a given round of ammunition. To do this, you’re going to need a drawing, or at least a very good guess, of the cartridge case itself. Keep in mind that cases change dimensions over their lifespan from trimming the case neck until they are spent brass; what we’ll be measuring here is the effective case capacity of an empty, unfired case.

For the example in this post, we’ll use 5.56x45mm NATO, comprehensive drawings of the case being available online. The one I’m using is found below:

Kfleb

As Andrew Tuohy once said, military specifications are very exacting.

From this drawing, I created a single part in SolidWorks representing the case itself. This is very, very easy, and simply involves drawing half of the cross-section of the case itself (axis line included), split down the center of the flash hole, and using the “Revolved Boss/Base” tool, select the axis of revolution, then the contour (which will be the cross section of the case). Be sure to uncheck the box next to “Thin Feature” before hitting the green checkmark that will give you your finished case.

selectaxis

Click the image to expand. It’s important to have the axis of revolution centered properly, or else your volume will not be right.

selectcontour

Select the contour to create your case. Notice that “Thin Feature” is selected; deselect it before hitting the green checkmark.

The finished case is useful for many things, but it’s not what you want to find the case capacity itself. To find the volume inside the case, go to the parts tree and right click to reveal an “Edit Sketch” button. Left-click, and you’ll be able to access your sketch again.

editsketch

Editing a sketch to find the case capacity will save you a lot of time. The errors you get from doing this can be unsettling, but with proper correction, won’t harm your work.

Now it’s time to cut away everything that doesn’t represent the case volume. Select the “Trim Entities” tool, then select the “Power Trim” option, to cut away the outline of the case, leaving only the case volume. Be sure to leave the volume of the flash hole intact, but not the volume for the primer.

powertrim

Power trimming is a lot easier than selecting each line segment and deleting it.

Complete the volume by attaching the contour to the axis of revolution, and then hit “Exit Sketch”. A prompt will appear; just select “Exit the sketch and rebuild anyway”. This will open another panel telling you there’s an error, just close it. Now, right click the Revolve tab on the part tree, which will have a large stop sign with a white X through it, and left-click “Edit Feature”. Now highlight the “Selected Contours” box, and select the case volume. Hit the green checkmark, save under a new name, and now you have modeled the case capacity of your cartridge in SolidWorks.

556casevolume

The case volume will be in cubic millimeters, which no one uses to measure case volume.

To find the capacity itself, click the “Evaluate” tab and select the “Mass Properties” option. To find the volume in grains water, simply take the number given for “Volume” divide it by 1,000, and then multiply it by 15.43. Our 5.56mm case model gives us 30.35 grs H2O case capacity, which is pretty accurate!

556casesw

The finished 5.56mm case model. You can embellish it with fillets and textures, if you like; though it’s usually best to keep a plain model like this one handy for wildcatting.



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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  • JumpIf NotZero

    experimenting with hypothetical wildcat cartridges

    Interest level meter: -100.00%

    As someone who enjoys shooting and training, I find the idea of wildcat rounds entirely backwards to any goals I might have. While yea, solid works is great at visualizing designs, I think even if r idea of “special” custom made rounds were appealing, if probably allow someone else to waste their time and money on the invention and prototyping.

    But good write up if you’re into it.

  • J Bowles

    That’s some sloppy SolidWorks work, do the world a favor and fully define your sketches.

    • J Bowles

      If anyone has to work on it aside from yourself it will be a PITA, it will make your life easier too. Use Smart Dimension tool to dimension your sketch, it will help keep it from exploding if you change it, it will also keep you from inadvertently changing the sketch by accidently dragging part of it.

      • I’m sorry you found the drawing I whipped up in five minutes unsatisfactory!

        • J Bowles

          It’s a really really good habit to get into, that’s all.
          Trust me I learned it the hard way.
          In fact it’s likely faster to throw a dimension on something, than it is to try a chase point and line locations around.

          • I fully define sketches before I export the model, if I’m doing CNC work. For finding case capacity, it’s unnecessary. No one is going to use that model to make anything; if the cartridge goes beyond the conceptual stage, you might export dies or a reamer drawing, but the actual cartridge model doesn’t get used.

  • Geoffry K

    Wow! Talk about making it more complicated than necessary. I just filled an empty 30.06 case with water and weighed it. 68.8gr. H2O. Just plug the primer hole and use the empty case as tare, then fill and weigh.

    • In the event that you either do not possess the cartridge case you are trying to measure, or it does not yet exist, your suggestion isn’t very helpful.

      The grains of water a case holds is an input for many internal ballistic programs.

      • iksnilol

        How does grains of water translate to grains of powder? Or is it not consistent to measure grains of powder due to different powder types?

        • You nailed it; they’re two different things. In particular, it seems to me Geoffry there doesn’t realize that if you try to fill up a case, especially a rifle case, all the way with Bullseye or some other fast-burning powder, you’ll end up with gun fragments embedded in your arm.

          Sure, if you’re loading something like .303 British using Reloder 15 or another slow powder, you can pretty much just top the case up to the neck, and away you go, but not all powders are so forgiving.

          • iksnilol

            What!? I thought you just take and fill up the case with whatever powder you, more pressure=more speed=more accuracy/power, amirite?

            /sarc

        • Doesn’t matter. What you are effectively measuring is the internal volume of the case – case capacity.

  • Todd

    Home reloaders are going to buy a seat of Solidworks, at $4500 (cheapest), for a feature as minimal as this?

    • Many, such as myself, have the program on a free student license. Of course you wouldn’t buy SW just to be able to do this, but if you are a student or business who already has the program, it can be very useful.

    • Man pippy

      Yep or bother learning solidworks in the first place for that minimal feature. Author should have used a free program like Sketchup/Rhino.

      • Many, such as myself, have the program on a free student license. Of course you wouldn’t buy SW just to be able to do this, but if you are a student or business who already has the program, it can be very useful.

        • gunslinger

          i know my AutoCAD has a “home use” license availble for when i’m not at work.

          and yes there are a number of other free CAD software programs out there.

  • J Bowles

    A more straight forward method in Solidwork to get find the case capacity might be to set the material to water(found under Other Non-metals), and just find the weight in pounds then divide by 7000, a conversion most reloaders know by heart.

    • In my case, I’m converting grams to grains a lot, anyway, so it’s faster this way for me. There are of course different ways to do it.

  • Ed

    Or just ditch this computer crap and read you reloading manual!

    • Thabk you for your opinion.

    • iksnilol

      Reloading manuals won’t help much if you are doing something that hasn’t been done before. Your options are either “computer crap”, making the case itself and measuring or, my favorite, going in blind and hope nothing blows up.

    • Hey, Ed, this isn’t 1964 anymore. Welcome to 2014, where computers are ubiquitous, essential, and far more useful “crap” than you might think. Your reloading manual is no longer essential, it now completes the computer.

  • Ben Warren

    And so, we discover that there’s no asshole greater than a reloader being shown a new way to do something.

    Nathaniel’s method isn’t the one I’d use, but finding case capacity for a new wildcat cartridge is a use for Solidworks I’ve never considered.

  • Alex Nicolin

    I did the same thing with a huge excel sheet. In fact, it’s much more than that. It estimates the minimum needed web and other walls thickness based on maximum pressure and the tensile strength of brass, then computes from inside out. I got 30.44 gn capacity for .223, which is also the maximum for production cases (it varies by manufacturer). After the calculation, I make the model in Autocad 3D.

    • Very neat! Care to share?

      • Alex Nicolin

        Yes, I can send you by e-mail the results. The spreadsheet itself is very user unfriendly.

  • N8

    For those that need a good CAD program that can utilize Solidworks files, take a look at Siemens Solid Edge. You can get a subscription for 20/mo through localmotors.com. It is crippled on some functions but all the major functions are there. I have drawn up a bunch of gun parts and other fun stuff for my 3D printer with it.

    • N8

      Posted this up because of the comment about solidworks costing 4500 smackers.

      • Guest

        Smackers