Protecting Electronics in Metal Military Surplus Ammunition Cans- Part 1, by J.T.

In this multi-part article, I am sharing a better solution for protecting sensitive electronics stored in metal military surplus ammunition cans. There are many references for making a Faraday Cage on SurvivalBlog. Some references go back as far as 2005. One solution was proposed in 2006 that required the removal of the rubber gasket on ammo cans and replacing the seals with a conductive material (stainless steel or steel wool). Other articles suggested the use of a galvanized garbage can that is sealed up with conductive metallic tape, or an open head steel drum, or using an old microwave oven and a metal cabinet. I have read and researched many great comments and feedback on the effectiveness of these EMP protection measures.

Previous Proposals

All of the proposed solutions are more or less effective. Some have been tested using some sophisticated test equipment that gives you some confidence that different approaches will be effective. But unless your solution is tested, then it’s still unclear if or how effective your solution will be when needed in the future.

The problem with many of these solutions is that each didn’t make it convenient to access. Perhaps it required the metal container to be resealed, while other solutions were sometimes big and bulky, or untested and may not be reliable in the worst-case scenarios.

The risk with these solutions is the reliability of the seal. Ensuring that the container has no openings is also critical. This would allow electromagnetic radiation to penetrate to the interior of the cage or container. After much research that yielded no comparable solution and the successful prototyping of this gasket design, I found this to be the right opportunity to describe how this gasket is made and installed.

The Solution

The solution is a very effective metallic gasket, using sheet metal and tinned copper mesh tape. The gasket can be made with readily available materials and tools, and it requires only modest mechanical skills. The gasket makes nearly any metallic military surplus ammunition can into a very effective faraday cage and container.

The cost of materials for each gasket is in the range of $8 to $15 dollars, depending on the size of the gasket and the desired “belt-n-suspenders” to enhance the effectiveness of the gasket and container.

My Objective

In the most recent design challenge, I sought a solution that was reliable, convenient, and would help to organize the storage of sensitive electronic equipment. Most people are unlikely to build or can afford a faraday room or closet to protect electronic equipment. Furthermore, I don’t have the space to accommodate several galvanized garbage cans or 55-gallon metal drums in our garage or basement. A solution that provides protection from radio frequency or electromagnetic interference while providing convenient access and meeting the waterproof and airtight requirements was the optimal and desired solution.

Organizing these solutions to make each interoperable would also be valuable. Finally, convincing our family members of the value of such a solution is another challenge. Any solution has to be something that the whole family will value. More importantly, it must be something where they won’t be intimidated to use the solution in an emergency or in everyday use.

The Beginning- The Ammunition Can

As a starting point, there are a variety of ammunition cans for consideration, including the big 120mm mortar cans. These mortar cans are 32” by 12” by 5½”. The 40mm can is 17½” by 10” by 6”. An 11” by 5½” by 7” option is the 50mm can. In my opinion, the best can is the 20mm variety; it’s not too big, not too small, and won’t be too heavy to move. But all of these ammo cans will accommodate a variety of sensitive electronics of all sizes and shapes.

The 20mm M548 ammo can is a personal favorite and has inside dimensions and usable space that measures 17 1/4″ by 14 1/16″ by 7 3/8″. Outside dimensions, with the lid attached, measure 18 5/8″ by 14 7/16″ by 8 3/8″. These 20mm ammo cans will easily accommodate a laptop and tablet with power cables, or an inverter up to 2,500 watts, or a small 20 to 40 amp solar charger and controller. It also provides a way to build a battery bank to run equipment like HF or VHR/UHF radios, on an inverter.

Directions For Building Ammo Can Gaskets

The following instructions and pictures describe how to make almost any ammo can resistant to the effects of Radio Frequency (RF) or EMP (Electromagnetic Pulse) interference, including the likely effects from a geomagnetic solar storm. The solution retains the well-known water resistant and airtight effectiveness of these ammo cans. These cans may be easily opened and closed without compromising the shielding effectiveness of the gasket.

To make these gaskets you will need the following materials, tools, and hardware:

1. A sheet of 18 to 24 gauge metal

Galvanized sheet metal will work, but you may also use standard hot rolled unfinished sheet metal. If you want to splurge, you could use copper metal sheets that will be more expensive, but you’ll have the “Cadillac of gaskets”. Don’t use aluminum or stainless steel sheets for reasons that will be more obvious in the construction of the gasket. The size of the sheet metal needed will depend on the size of gasket you are making. So I suggest that you start by purchasing a 12” by 24” sheet, which are readily available at big box hardware stores. This size will make two or three gaskets, including at least one for the big 20mm cans or the 120mm mortar cans. Cost: $4 to $12 per sheet.

2. 3M Scotch 24 Copper Mesh Electrical Shielding Tape that’s rated from 32 to 220 Degree F.

The tinned copper tape mesh roll is 15′ long by 1″ wide. This product may be hard to locate but is generally found on eBay for $12 to $25 per roll. Other online retailers also carry the product. The product is a very thin, flexible tape made of tinned copper mesh. The construction of the tape makes it easy to solder the product to a properly prepared metal surface. In the early prototypes of the gasket, I was unsuccessful in using flat tinned copper braid. This product is generally too thick and makes it difficult to seal the lid to the can. In future prototypes, I may try to split this braid and use only a single layer in lieu of the mesh tape. Cost: $12 to $20 per roll which is usually enough to complete two or three gaskets.

3. Straight Cut Sheet Metal and Aviation Snips.

Cost: $5 to 16. However, you can find these at garage and estate sales, or you might borrow a pair of snips from a friend. You might also ask a local heating and ventilation contractor to cut a piece of sheet metal to size for this project.

4. Lead/Tin Solder, Rosin-based

I prefer to avoid the acid core solder that will corrode metal if not thoroughly cleaned. However, some would suggest a good acid based flux and 50/50 solder using a high wattage iron works the best. I’ve also had good experience with thin 60/40 solder with rosin paste flux. However, don’t use a torch of any kind. It will degrade the galvanize and will never allow the solder to flow properly. Also from one bad experience, I can tell you it will burn up the mesh shielding tape. Cost: $3 to $5 for flux and solder.

5. Soldering Gun or Pencil

You will need at least 100-watt soldering iron, and it is better to have more heat than less if your sheet metal is of a thicker gauge. Cost: $25 to $55, depending on the wattage and features. However, again you might find a friend with an iron and build some gaskets together to avoid this expense.

6. Phillips Head Sheet Metal Screws

It is preferable to use steel screws in size #6 x ¼” or #8 x ¼” that are self-tapping. These can be found online or at the local hardware store. Alternatively, a #8 or #6 Philips head counter sunk screw with a finishing washer will make a nice enhancement (see picture). Important assembly step: the screws can’t be longer than 3/8” and the preferable length is ¼”. It may be hard to find these steel screws, but keep at it. These lengths are the only size that will work to properly secure the gasket. You will also need a power drill and a 3/32” steel drill bit for the pilot hole to start the screws. Cost: $1 to $3, depending on the source and use of finish washers.

7. Sand Paper or coarse Steel Wool

The sand paper or wool is to remove any paint where you will need “bare metal-to-metal” contact. A dremel tool is a handy way to remove paint in small areas and will have many other uses around the workshop. Cost: Minimal (for sand paper or steel wool). A starter Dremel Tool kit can be found for less than $25 or borrowed from a neighbor or friend.

8. M548 20mm Ammo Can

This can must be in good condition with no penetrating dents or any bent or dented edges that would prevent a good contact and seal with the rubber gasket on the lid. Finally, it must have good seams that are not rusted or split or badly corroded. For this project and the several ammo cans that have been enhanced with the shielding gasket, these cans are painted with flat paint, using different colors (black, brown, and green) to distinguish what’s in the various ammo cans. (I have more on this later.) Cost: $5 to $45 per can, depending on the size of the can, condition, and cost of shipping. I am able to consistently find these 20mm cans at gun shows or military surplus stores for $35 dollars. They are usually in very good condition.

SurvivalBlog Writing Contest

This has been part one of a four part entry for Round 71 of the SurvivalBlog non-fiction writing contest. The nearly $11,000 worth of prizes for this round include:

First Prize:

  1. A $3000 gift certificate towards a Sol-Ark Solar Generator from Veteran owned Portable Solar LLC. The only EMP Hardened Solar Generator System available to the public.
  2. A Gunsite Academy Three Day Course Certificate. This can be used for any one, two, or three day course (a $1,195 value),
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  4. DRD Tactical is providing a 5.56 NATO QD Billet upper. These have hammer forged, chrome-lined barrels and a hard case, to go with your own AR lower. It will allow any standard AR-type rifle to have a quick change barrel. This can be assembled in less than one minute without the use of any tools. It also provides a compact carry capability in a hard case or in 3-day pack (an $1,100 value),
  5. An infrared sensor/imaging camouflage shelter from Snakebite Tactical in Eureka, Montana (A $350+ value),
  6. Two cases of Mountain House freeze-dried assorted entrees in #10 cans, courtesy of Ready Made Resources (a $350 value),
  7. A $250 gift certificate good for any product from Sunflower Ammo,
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Second Prize:

  1. A Model 175 Series Solar Generator provided by Quantum Harvest LLC (a $439 value),
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  1. A Royal Berkey water filter, courtesy of Directive 21 (a $275 value),
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  7. Montie Gear is donating a Y-Shot Slingshot and a $125 Montie gear Gift certificate.,
  8. Two 1,000-foot spools of full mil-spec U.S.-made 750 paracord (in-stock colors only) from (a $240 value), and

Round 71 ends on July 31st, so get busy writing and e-mail us your entry. Remember that there is a 1,500-word minimum, and that articles on practical “how to” skills for survival have an advantage in the judging.


  1. I know that there is a lot of bad information on the ‘net when it comes to EMP effects and mitigations. What is not clear to me, is why an ordinary “used” ammo can is not an adequate protection against radio frequencies, all by itself? At one point I tried calling a cell phone inside a can and was able to ring it until I sealed the lid… then silence. To my way of thinking that achieved the desired goal, even without taking the extra steps of improving the conductive properties of the gasket. If someone has done more sophisticated testing, I would be interested in seeing that before spending yet more money on a fancy gasket.

      1. Not quite every point. If you can’t get at least a quarter of a wavelength into the conductor, no significant energy will be absorbed. That’s why I’m unconcerned about the gasket — we’re talking about less than a centimeter.

        Also, remember that radio waves are line of sight, so to get around the gasket, the signal has to be a short enough wavelength to reflect off the can, reflect again off the inside of the lid, bounce back and forth there (losing energy on every reflection), make the 180 at the lid, and then reflect back through the slit greated by gasket into the case, and then, after all that, find a conductor to dump its energy into in the contents.

        That’s why I’m not worried about the gasket. And all of this assumes that the lid is not electrically bound to the case, in which case skin effect is going to prevent the bouncing, and we don’t even get that far.

  2. New metal paint cans can be had at various hardware stores. Once the sealant is cleaned off where the lid meets the can I believe that these will make a servicable faraday cage

  3. I used a metal trash can and instead of metal tape to seal the lid, I used thin strips of aluminum foil doubled over the top edge of the can itself and kept adding foil until radio signals and cell signals would not reach inside….Then I marked the lid and side of the can with a marker to orient the lid and can the same each time the lid was removed and replaced….easy to get into…however you may have to add layers of foil from time to time if you are in and out of the can very much…as with most methods we probably will not know if they are effective unless we have an EMP which I hope never happens…

  4. I would like to see some test results. For example:

    Personally what I’ve done is used some of those Christmas Cookie tins (large metal box, metal lid, no paint on the inside edges) to store some radio equipment, USB drives for backing up data, and more.

    Is this any better or worse? I don’t think any of us really know.

  5. If the lid overlaps the container edge it will shield out radio waves. Case in point: My I-phone fits nicely inside a Bass-pro gift card tin box. The tin box is enameled steel, inside and out. The phone does not respond when called, even though I did not put any tape or foil around the edge. Same for a galvanized steel trash can. If the lid fits tight all the way around, put a battery operated FM radio in it (tuned in, and loud) and close the lid. If it’s protecting,
    the radio will fall silent.

  6. One of the things this article seems to be missing is the concept of wavelength. Radio waves are relatively long waves compared to light. I’m not sure what wavelengths we’re trying to block here — to get around the lip of the can, the waves would have to reflect back and forth between the lid and the can and then around the tiny gap created by the compressed rubber gasket.

    So what are we talking about here? Sub millimeter? Let’s say it’s .2mm. We’re talking terahertz frequencies at that point, and you’re just not going to have the ability to build up much of a pulse at that frequency. This is trying to solve a problem that doesn’t exist. Teraherz radiation is absorbed by things as diffuse as clouds and fog, so by the time it goes through atmosphere, then your roof, then your ceiling, and whatever else is in the way (closet door, for example) it’s going to be so weak it never had a chance to start with.

    More likely, if there is an improvement, it is entirely from electrically joining the lid and the can. The thing is, you can do the same thing with a jumper wire soldering them together, or a few pieces of conductive tape. (You don’t even have to seal around the whole case, just a spot or two to connect them.

  7. ALL VERY interesting, but the devices being used in these “tests” are trying to detect a signal measured in pico-volts. A 50KV to 100KV signal is a whole nuther animal. A popsicle stick will insulate against a 120 volt AC power line, but would you trust it handling a 100,000 volt line? It is, after all, electricity. My sources suggest a cage thickness of .100″ of steel. Thinner gauges will certainly help but we won’t know of sure what the threshold of failure is until That Day. 12 gauge thickness is .109″. The utility of most of the items we are trying to protect will probably not matter much. Radios, cell phones, all worthless when their supporting infrastructure dies. Smart phones can hold files containing information, but their value as a communicator is lost. The many comments on the folded lids are solid. Even radios placed in file cabinets were protected during simulator tests, despite the generous gaps in the drawers. Small devices are inherently more robust….they have very little antenna footprint, and are not connected to the grid. i-Phones are shown to work after exposure to threat levels of insult with NO cage around them. Depends on the architecture of the device. It’s far cheaper to design in surge suppression than to retro-fit protection.

  8. It is, after all, electricity.

    That’s just it. It’s not electricity — it’s EM radiation. It doesn’t work like electricity — it works like as much like light (which is also EM).

    You aren’t trying to ground it. You aren’t trying to insulate against it. You’re trying to use skin effect to have the EM pass around the cage.

    We’re doing things like electrically joining the pieces so that the EM sees it as one piece. You’re absolutely right about the filing cabinets, because the overlapping metal sections (where the drawer fits into the casket) create radio traps that reflect the signal back out rather than allowing it in.

    It’s a real esoteric subject that I really only wrapped my head around once I started making antennas (which is the opposite task, trying to absorb the EM rather than shuck it) and I can’t come up with a good, plain english analogy. The best I’ve come up with is that you are making a boat to ride out the EM storm, not trying to build a wall to stop it.

    1. Everything on the grid will be fried because a magnetic field passing through a long wire is a huge antenna. Antennas convert magnetic radiation into electricity.

      The objective is to not allow it to become electricity…

      If you have two 2-way radios in an effective Faraday cage, you can have comms, post event. If you have shortwave comms, you can communicate with parts of the globe which were not hit.

      So, if I have a used ammo can with reasonably good electrical contact between the lid and the base — does that construe an adequate Faraday cage? What other tests can we use to determine the effectiveness … beyond the basic cell phone ring test?

      1. What other tests can we use to determine the effectiveness … beyond the basic cell phone ring test?

        Like everything in testing, the devil is in the spec. What are we testing? Wavelengths shorter than the box itself we can discount, because if you can’t get a full length into it, it can’t dump any energy. SO right off the bat, we can ignore any wavelength longer than 40cm. That puts us at about 800mhz. The trouble then is getting the antenna right. Once you are into terahertz lengths, it doesn’t matter, because we can’t even measure frequencies that high digitally (we have to do tricks like dividing the freq, so we’re just adding more complexity.)

        So we’re talking really about gigahertz freqs. The only thing most people have that operates at that level is cell phones and wifi. That suggests that the cell phone ring (about 900mhz depending on your carrier) and wifi (2.5-5 ghz depending on your router) are our best tests. Put your phone on Pandora on wifi, and stick in the box. If the music stops at the end of the song, and you can’t ring your phone, that’s probably the best test you can manage. If the energy is low enough that it’s in the noise, it’s low enough to not wreck your electronics.

  9. It is useful to note that a nuke provides a FULL spectrum of EMR from the shortest to longest of wavelength at very large magnitudes for a very short time. I believe that covering my SW radio and Laptop and Hard Drive in bubble wrap pouch and then Aluminum Foil and then again bubble wrap pouch and then again in Aluminum Foil to be place within the metal box of whatever design will provide the best protection for my necessary backup electronics. I keep my backup electronics in a Job Box which has been altered to seal it up and lined with cardboard. I back up once a week and consider all my desktop and day to day toys to be sacrificial- after all, what do I know about the timing of a mushrooms appearance.

  10. Because it takes so little extra, I double up the protection hoping to have nearly 100% success. The large pop corn tins sealed with the aluminum tape are my favorite. The electronics are contained in a thick insulation material, and are wrapped in aluminum foil with the seams taped with aluminum tape, and is protected from the can with another thick layer of insulating material. Desiccant is used inside the packaging containing the radios.

  11. Once we are hit by an effective man made EMP and our country is foundering (by N Korea?), there will other enemies (Russia, China?) who will want to keep us from recovering. Therefore there will be follow up EMP strikes. These will probably be weeks and months later. What then? It will be risky to have any equipment out of it’s protection.

  12. Openings and gaskets are an important aspect of having an enclosure that will shield against EMI and the shielding effectiveness (SE) is a function of frequency.

    1 simple 1 page reference on openings:

    And an 88 page reference for the phd electrical engineers:

    And if you a glutton for punishment, a 200 page reference on using gaskets for shielding.

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