You have to pursue at least four parallel tracks - your engineering education and then real world practice is one, the second is learning every single detail you can about the history and design of firearms that has already gone by, the third is to get more hands on experience shooting all types of firearms and hand loading than anyone you ever heard about, and the fourth is learning how to run a mill and a lathe and then building guns yourself.

You need to study everything you can about mechanisms, and then practice designing mechanisms.  Get access to the patent database and study the guns that were patented over the last 150 years or so.  Learn about the real differences and applications of materials and how to select them.  You have to learn to think outside the box to design a new firearm that is successful.  Buy guns and build guns.  Put all your prejudices about firearms you don't care for aside, no matter the reason, and study them all, keeping your mind open to new ideas that will synthesize from this work.

Maybe the most important is to adopt an attitude that you are more interested in the facts than the hearsay and old wive's tales about firearms and shooting.

You're thinking about a career with limited full time possibilities.  The US alone is full of hobbyists that can build fine firearms from scratch to their own design.  But, if you bring a plate full of experience, you have a shot at consulting work for various manufacterers.

There's more, but I have to go now.