Hard? Depends what you think hard is. If 100 people tell you no that you can't do something do you keep trying? If you fail at something over and over again go you give up
If you believe that failure is a good thing, by all means go for it...you will be a failure at this on a daily basis. Are you Ok with that? If not, then No, you won't be able to do this. However if you're ok with failing everyday you'll do quite well.
Aside from the legal aspects of making a gun and marketing it, the design work is a daunting task that is 98% failure...unless you're just copying another design.
I worked for Briley years ago and Jess Briley, the owner was working on his own shotgun design. A gifted individual with talent and knowledge many in the industry would like to have. I don't think he ever was able to achieve the goal for his design.
When I worked for Other Stuff Gunsmithing we were working on several designs, which never came about. This is either due to people just not wanting it, incompatible issues; tooling problems...the list goes on and on. We did have one design that worked out, only to have it basically stolen. It was the longslide 45 that Mitchell Arms was offering.
That was to be our design from our company...when it was finally marketed, we got nothing.
Point, is. Yes, it can be done but it’s a long road to get there.
Really what you need is great attorneys and a lot of money. Oh, and a knowledge that it's a job of passion, not one to get rich from