Designing a Firearms Magazine
This tutorial will cover design of a slightly above basic firearms magazine. The concepts touched on here cover both simpler designs, and more complex.
Step 1: Choose a cartridge
In order to design an effective magazine, the cartridge required must be known to the designer. While certain elements of magazine design carry over between cartridges and requirements, exact dimensions are key.
You can gather your dimensions from physical examples with varying degrees of success, but for exact standards I recommend the Sporting Arms Ammunition Manufacturers' Institute or SAAMI for short. This organization provides the standards for US companies to aspire to while manufacturing ammunition, and gives this information out to the public, on the internet. While European counterparts exist, I live in the United States, and any cartridge common enough for me to design anything for is covered by SAAMI.
Step 2: Design Considerarions
While much of the ideas behind designing a magazine are "standard" between designers due to their functionality, there are still important decisions to be made as to the exact construction of your magazine design.
Double Stack vs Single Stack
A single stack magazine is simple to understand and design. It is comprised of a single column of cartridges, one on top of another, propelled upwards by a spring. It is as easy to manufacture and design as possible.
A double stack magazine is slightly more complex. Two complete stacks of cartridges ride side-by-side, offset vertically from each other by one half the cartridge diameter. Feeding these cartridges out of the magazine is either accomplished by sorting the two stacks into one, or feeding from either stack alternatively.
Full Contact vs Minimized Contact
You can design a perfectly functional magazine that is a simple box that contains cartridges and moves them towards a feeding device. However, contact between the sides of the magazine and the cartridge body generates friction, necessitating a heavier spring. For lower capacity, and usually single stack, magazines this is not a problem. However in higher capacity magazines, magazines that require a fast response time, or to minimize the possible effects of dirt, less contact with the cartridges is preferred. This is usually done with a series of inwards ridges that control the horizontal motion of the cartridge whilst minimizing the contact with the walls. These also have the added benefit of increasing the rigidity of the magazine.
Curvature / Slant
Many magazines are curved or slanted in some way. This is to accommodate the taper of each cartridge, rims, a handgun's grip, and so on. The designer must figure out ahead of time what features are important for both the function of the final product, but also manufacturing concerns and the overall lines and aesthetic of the firearm it will fit into.
Step 3: Example in practice: a 9mm SMG mag
In this example I'll be designing a double stack, single feed magazine for 9mm Parabellum, much like the STEN or MP40 magazine that have been copied and modeled time and time again. While this magazine design is not very good, it is quite functional and easy to make.
Instead of the complicated reinforcing piece atop the head of the magazine, mine will simply be made of a higher grade, heat treated steel sheet.
Step 4: Body Profile
Firstly, I obtain the maximum outer dimensions of the cartridge I'll be using from the SAAMI performance standards document relevant to my needs. Using the maximum dimensions means that all ammunition should fit in our magazine, unless it is improperly manufactured. This also gives you a starting point for your tolerancing.
I start a new sketch on the top plane and lay out the critical outer dimensions for my cartridge. Any wall built to contain the cartridges will have to interact with them at these points, since they are where the protrusions are.
Here I've sketched the main profile of the magazine. It is basically a box, with rounded corners. Don't worry about the fillet towards the front intersecting with our box - Bullets usually aren't squared off like that, but instead come to a point, so there can be a little bit of intersection. When you take that thought into consideration, you'll realize that the front of each bullet is riding on the magazine by an extremely small point, which reduces friction.
At the rear of the magazine is a bump inwards. This will serve many purposes. Our magazine will be stronger and more rigid, but this will also provide a place to weld the gap together, since the magazine will be formed from sheet steel. The cartridges will ride on this bump, reducing contact against the rear, and hence friction. Also, any dirt that enters the magazine will fall to the bottom through the gap to the rear of the cartridges.
The walls of the magazine are defined as coincident to the two points that make up the sides of the cartridge I defined earlier. These two points are the base of the cartridge case, and its neck or shoulder.
But how wide should a double stack magazine be to perfectly accommodate two stacks of cartridges? Remember they are offset, and sit nested within each other, so it's not two times the diameter. Actually, this is quite easy to figure out with analytical trigonometry, but all designers take shortcuts. Simply multiply every horizontal dimension of the cartridge being considered by 1.866 to obtain approximately the correct width.
Here I've done just that. I've also mirrored my profile across the vertical axis, creating a closed shape. You'll notice the slight overlap where our cartridge boundaries cross this axis - this is how much the two stacks will overlap.
Now I'm going to extrude this as a solid shape. If you want to make your magazine curved, you'd use a sweep operation here, but it's not strictly necessary for a decent amount of 9mm Parabellum cartridges. The British Lancaster submachinegun was issued with a totally straight 50 round magazine that worked just fine!
How long the magazine should be is up to the designer. First you need to figure out how many cartridges you need to hold. In my case, I'll pick 20. Divide this number by 1.866 and the diameter of your cartridge (this is also a horizontal dimension since cartridges are radially symmetric) to get the approximate length of the stack of cartridges.
( 20 * 0.394 ) / 1.866 = ~4.22 inches in overall length.
Depending on your exact design features, your spring may vary. But I usually like to multiply this length by 1.5 to leave enough room for the spring. Due to how this is being laid out in Solidworks however, it will be easy to change, so don't worry too much.
~4.22 * 1.5 = ~6.33 inches in overall length.
Step 5: Feed Lips
Now that we have a solid profile for our magazine, we'll create the feed lips that retain cartridges and direct them towards the barrel of the gun.
For this design, I'll be using the single-feed system, which basically combines the two stacks of cartridges into one. This makes designing the feed system of the firearm extremely easy, while sacrificing ease of loading and reliability, to a minor extent.
A cut is made from the front of the magazine to reduce the diameter from that of 1.866 times one cartridge, to just under one cartridge. It needs to be slightly less than the diameter of the cartridge so it cannot escape vertically out the top of the magazine. The observant will notice that my cut is made straight, and the cartridge is tapered - this can be taken advantage of to introduce an angle into the cartridge without explicitly cutting this into the magazine lips. This angle can also be avoided by tapering this cut across the axis of the cartridge as well, however, I am not this anally retentive and design my feeding systems to accommodate a large amount of angular variance.
So how long do these cuts need to be to sort the stacks from two down to one? This is a tradeoff. The shorter the cuts, the larger the magazine that can be retained in a shorter space. The longer, the less stress on the magazine body, and outwards force generated by cartridges, and by extension friction. Mitigating friction is generally best, so I usually utilize a length twice to three times the diameter of the cartridge.
If your magazine does not require any additional angle to the feed lips, you can skip this step. However in some systems, additional angle may be required. This is accomplished by simply cutting an angle into the top of the magazine.
Now, we simply introduce a radius to the corners of the magazine's top to form the feed lips. This radius is made to be similar to the diameter the top of the feed lips are cut to, to take into consideration both strength and contact against the cartridge. Remember to not use "tangent propagation" to generate this geometry.
The top of your magazine should look similar to this. Here I have rounded the edges to take into consideration the sheet metal forming that will be done to this area.
However, the magazine still needs a hole in the front of the feed lips for the cartridges. This is where ammunition will be fed forwards and outwards by the firearm, and inwards and backwards by the user to load it.
This hole will also help to dictate the angle of the cartridge as it exits and is pressed between the feed lips. Experienced users can take advantage of this effect to guide the cartridge more precisely, but here we will make a simple hole.
How long should this hole be? Generally, the feed lips need to come forward enough to support the cartridge fully, whilst not interfering with feeding. Ideally, the magazine should stop touching the cartridge once control of it is handed over to the firearm's feed ramp and bolt edifice. However in practice, this is generally just about the center of mass of the cartridge. Many 9mm magazines have feed lips that extend to about this area, so mine will extend to the approximate center of mass as well.
The magazine profile is then shelled outwards a suitable thickness to approximate a sheet metal forming. The cut for the cartridges to be fed outwards, as well as the bottom, are left open.
The feed lips must have an opening in the top to be fully realized. This allows the bolt to reach between the lips and scoop out a cartridge, pushing it forwards. First, a plane is created in line with the angle of the feed lips. Then, we sketch on this the profile that we'd like to cut out. The bolt will ride within this profile, and the upwards force experienced by the cartridge stack will be against this cut, so consider it carefully.
The bottom of the cut is rounded. It doesn't have to be fully round, however this reduces stress, and my part will be hardened here. You want the cutout to be wide enough that feeding is reasonable and friction against the cartridge will be minimal (since this is the area of maximum force) while still having enough "meat" to ensure the lips will not spread over time and allow cartridges to be vomited out of the magazine's top at an inopportune moment. The exact vertical location of the cut also depends heavily on the feeding system of the firearm it is being designed for.
The magazine's feed lips should now be fully functional.
Step 6: Final considerations
The rest of the features of the magazine, such as the rails on the bottom that retain the floor plate and close of the bottom, as well as whatever system of stops and catches that secure it into the gun, are simple mechanical aspects that should be decided by the designer based on their skill level, understanding, and requirements. Therefore, I will leave these aspects up to the designer.
Best of luck to you.