Armorers are generally only repairmen, and know how to follow a disassembly manual, and replace a part, or modify a firearm with a rail, etc.
A Gunsmith, on the other hand, has been taught everything about firearms, to the engineering level, including ballistics, design, fit up, safety, or the whole nine yards. They are at home with working with wood and steel, and now plastics. A good majority can design and build a safe firearm from scratch.