To reiterate/add to what MRM says, IMO you absolute need to set up a legal entity. While in my experience Sub. S/LLC's do not offer any significant tax benefits for the small business/sole proprietorship, they are worth their weight in gold to insulate you from personal liability. Just make absolutely sure you keep it totally segregated from your personal business so as to not allow the corporate veil to be pierced. If you were opening a snow cone stand I'd wait and see how the business goes before going to the trouble of setting up a business entity. But you're dealing with firearms...suppose you refinish a stock and then some peckerwood takes it out and fires it with a full barrel of mud. When his survivors sue you personally for your "negligence" you'll undestand why.
Even if you were culpable, any recovery would likely be limited to the LLC's assets.
Sounds like a great and rewarding venture on many levels. Good luck.