Having to open ammo boxes to see what's inside is inconvenient and it allows humidity inside the can, which can be detrimental to the ammo. A piece of masking tape can be used as a simple and easy label, but it's prone to falling off and it doesn't look very good. Stickers are available, but they can be difficult to remove when your ammo inventory needs to be moved to a different can.
A stencil is a simple, permanent ammo can labeling solution - and it looks better than other solutions (in my humble opinion). One stencil can list all the calibers in your inventory, so the same stencil can be used on all your ammo cans. Simply make a mark in the stencil area next to the caliber(s) stored in the can. You can store multiple calibers in the same can without having to use multiple stickers or labels. A Sharpie marker can be used for marking the stencil, and the marks can be easily removed with rubbing alcohol as your ammo inventory changes.
While it's possible to make your own stencil from individual letters and numbers, I recommend having the stencil created in one piece. Laser stencil cutting machines are the proper tool, but their cost makes them uneconomical to produce a couple stencils. Fortunately, by searching the internet you can find people who will create custom stencils for a reasonable price. My stencil was created by Craft Box Design Studio on Etsy (did a great job for an affordable price - highly recommended).
I created a svg file by laying out the stencil design in GIMP (free software), then exporting to a svg file. You can also lay out the design in GIMP, export to an image file, then convert the image to svg using Inkscape (free software). The laser cutting machine can use the svg file to create the finished stencil.
Most people will brush paint onto a stencil, but I found it much easier to use spray paint. The spray paint I used was Seymour 620-1443 Industrial MRO High Solids Spray Paint. It covered well and dried fast.
After taping the stencil to the ammo can, I masked off the area around the stencil. I sprayed on a light coat, let it sit for about 10 minutes, then removed the masking and stencil. After 10 minutes the paint is dry enough so it won't smudge when lifting the stencil, but still wet enough that it won't peel up with the stencil.
The Seymour spray paint color was Equipment Yellow, which is very close to the yellow used by the military. I believe the yellow stencil color used by the military on ammo cans is (was) "FED-STD-595/33538" international yellow, flat sheen source (page 19). It seems to be a mix of mostly yellow and a little red. The hexadecimal color code varies from different sources, but #F8A00D should be close. It seems the military has switched from yellow stencil / stamp to white printed labeling. Classic yellow is my preferred color, and of course you can use any stencil color to suit your personal preference.
My ammo cans came from Clean Ammo Cans. Very nice cans and good service.
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Stencil image with dimensions (png file)
If you have questions, comments or suggestions please contact me at kevin@kevinsworkbench.com