There is actually very little problem with the feds for someone wanting to start a legitimate business. Local hassles may be a bigger problem.
If you have a properly zoned building, evidence that you have or will obtain machines and tools, and have some expertise, the main problem may be the same as any new business - getting customers with money to buy your product. You will also need insurance (hard to get), capital, and a source for tooling unique to the gun industry.
If you want to manufacture Class 3 firearms (e.g., machineguns), there will be not much more fed problems except you may need to show some interest from law enforcement or military, since sales to individuals are now banned. BATF will generally not bother a Class 3 manufacturer if he does a few guns for his own use, but they consider all the licenses except a Collectors License (C&R license) as business licenses, not licenses to engage in a hobby.
Note, though, that unless you want to make receivers or frames from scratch, a manufacturers license is not required. To do custom work or build guns on existing receivers/frames, only a dealer's license is required to work as a gunsmith.
Jim