It really depends on the gun you want to build, how fast you want to build them, and how many parts you are willing to buy rather than purchase.
Figure on 2 to 3 million dollars of assorted equipment (Metal cutting, heat treating,finishing), 10k to 30k sf of light industrial zoned buildings, a half dozen manufacturing engineers, 3 or 4 machine maitenance guys to keep all of it running, and a hellacious problem finding employees at all.
Oh yeah, a customer base to keep the cash flowing, and a gun to build
Most of the newer gun companies (kel-tec, Kahr) started as contract manufacturers, and started making guns.