As an employee in a shop that specializes in custom design jewelry, I believe I have a take on what a custom shop's responsibility should be. It is our practice first to work with a customer until they are completly satisfied. The customer is not happy, we work until they are.
To avoid errors ,the shop having the experience to know every aspect of the custom work, should give back to the customer a complete description of the work that will be done, and the work that will not be done. The gun will be unfinished? The paperwork will say so. Target barrel? It will be written down with copy to the customer before starting the project. Every detail of what will be supplied or not is given. For example, with jewelry, we can provide a detailed 3d image of the product and accurate dimensions before starting. Any hint of arrogance should be avoided. If the customer is not happy, our question is, what can we do to make you happy? Guns, like jewelry, are luxuries, and so both are in the happiness business. No one needs our product, so it should be perfect.
YMMV, but that is my take on it. No one should ever leave a custom shop unhappy. If they do, the shop screwed up.