This category is one of the largest, and contains an entire section on manufacturer manuals for specific firearms. This category is not just about guns, there are subcategories in archery, air guns, blades, and other weapons.
There are user manuals, gunsmith help, reloading data. While information should be freely shared in a free society, actually applying some of the knowledge in this section would constitute a crime.
An example would be the automatic conversion data. If you do not have the appropriate permission from the government making a firearm operated automatically is a very serious offense.