I'm fairly new to blender, and I created a gun I would like to texture. I am using online videos to help me UV unwrap my gun, but when I try to unwrap the seamed pieces individually, this pentagon shows up. I don't really like it. Any help on how to get rid of it would be appreciated. I think it came from me clicking "Follow Active Quads" in the unwrap menu, but I'm not entirely sure.
The first thing would be to see if the mesh has a non-square face (ie, with 5 vertices) and fix it if it does. If it does and you want do be lazy, simply add one vertex in the middle and use that to make triangles.
(Faces that are made of more than four vertices are known to cause problems in weird places, so making a habit of removing them and keeping a good topology will make things easier. I think blender for the most part doesn't have a problem with it except for edges and faces that do have the same location.)
The best way to fix the uv map is probably to make a new one. I would recommend you first use it with a checker texture to find any weirdness first, so you might discover any other problems you might have missed.