I have worked on some projects with the end customer being a military organization. None of the projects were strictly “offensive” devices, so I could somewhat justify that they could be used for protection of our armed forces as others have said.
Unfortunately, I don’t really get to choose what types of projects I work on. I’m pretty sure I’m not alone here. Working for a small firm, you pretty much have to take whatever work you can, because staying flexible is the only way to stay afloat.
The type of project that I would find really uncomfortable to work on would be if I had to design a “military-grade” product for a consumer products manufacturer in the firearm industry. Some of these companies are making serious weapons and accessories that really have no viable use outside of combat situations, but they are only available to regular consumers. And they don’t even attempt to hide it by marketing them toward hunting, target shooting, or personal defense. That’s the stuff that scares me the most.