First, I would suggest that you review the job ads on the major commercial firearms manufacturers to see what kinds of skills they shop for. Mechanical and process engineering are generally big 'high level' hot buttons for the design or manufacturing side, respectively. Material properties, structures, and logic will be important study areas within these disciplines.
Remember that a firearms manufacturer will spend far less of their budget designing new weapons as they will on the fixtures needed to make them. Production and support infrastructure are always far more heavily funded than pure R&D. Working for the manufacturing side of the house may seem less sexy, but it'll keep the family fed in times when there is no other sexy work to be had...
In addition to looking in the commercial space, I would suggest that you also look to the larger defense contractors that make kinetic weapons systems. The experience that you'll get in the engineering and production lifecycle from such employers will be invaluable later in your career.
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ETA - if you can get into the summer intern programs with any of the defense contractors, you will have a huge leg up on other candidates come graduation time.... I can tell you from long experience that the two biggest things that get a foot in the door for a new grad are the perceived rigor of the school you're attending (e.g. MIT is better than the local community college) and the presence of viable intern experience. The former is what it is and you cannot change it, but the latter is completely within your control.
You can PM me if you want to discuss this further offline.