I've got specialties that I can handle just about anything coming across my bench (AR15/M16, M870), am well equipped for others (1911s, Glocks, GP100s), and others that I can probably work on since I have the basic tools and knowledge to fix reliability, replace parts, or tune with a bit of work.
I started my gunsmithing career as a specialist only and gradually opened it up over the years. I've told this to many aspiring gunsmiths: I built and sold hundreds of custom firearms, but the number one gunsmith job that keeps paying my bills are the simple detail cleaning and scope mounting.
Oh, and if you ever have an exposed spring, wear eye protection. Seriously.