Acrylics dry faster. When dry brushing use a retarder medium, fixes the issue. Just remember not to use more than the stated %/volume.
Vallejo gunmetal does sparkle a bit. The AK Ammo version is better. mix 1p gunmetal, 1p flat black primer, 1p retarder. Remember metal is shinny. If not, then it's oxidized. That is a separate layer. Aluminum does not oxidize very fast. So a fresh scratch will be shiny. Steel oxidizes quickly, so you can/should add layers to it to tone down that shine. Sounds like the issue is more trying to get a quick fix, were a few more layers are needed for what you want.
A trip to a local steal yard with a camera is a fun way to get some real reference. Also a trip to the local heavy equipment repair/scrap yard. Steal does different things when exposed or worn is different ways.
Or...buy "real" metal paints. They tend to mimic the oxidization.