>>7662299>>7666169>>7666232LA stuff can be a bit finnicky if you aren't a gun-autist like me, but thankfully, most guns are flat black or gunmetal. For matte black stuff, like polymer, Citadel Lahmian Medium is a decent way to hand brush that kind of finish. And Citadel Leadbelcher is good for gunmetal, even better if you use a Nuln Oil wash and an Abbadon Black dry brush around the muzzle.
Reference pictures are usually pretty easy to find for guns, and if you ask /k/'s QTDDTOT thread about a specific gun, someone WILL have it.