I have questions about firearms, in particular about the functioning of a "flintlock", but really I'm a firearms fan and that takes me to many questions about all kinds of weapon styles. I also have questions about ICBM, and many more "war" related machines and apparatus. I think my questions are tecnical in nature and refer to how a machine works. So, should I ask them in engineering stack exchange?
Engineering Stack Exchange does indeed have a firearms tag. The suitability of your question really depends on what particular aspect of the weapon you're asking about. Three of the four are about the mechanisms of a firearm; the fourth is about a physical effect inside a gun. If your question is along those lines, I think Engineering would accept it.
If the question is unrelated to the engineering aspect of a firearm, I suspect it would be poorly received, and likely closed as off-topic.
There was once a Firearms Stack Exchange, four years ago, but that closed due to a lack of activity.
For larger-scale weapons, though, Outdoors wouldn't seem to fit. As mentioned already, Space Exploration might fit some ICBM-related questions. It's also worth noting that questions about the physics of explosions might fit Physics.