You can't select any weapons that you don't have the research to make yourself.

For example, you can't give units bolt-action rifles without having gunsmithing researched, or autopistols without blowback operation.

Some weapons (usually from mods) will show up in spite of that, usually because you can't make them and/or they don't have associated research, so I can see how it might be confusing.

If you *do* have the techs researched and still can't see the weapons, I don't know what the problem might be. Sorry.