TBH i would start by learning CAD and undertsanding the diffrent coordinate systems. Polar,Absolute, and Incremental. It helps to be able to visualise and oriantate in a 3d world for any type a cnc. There are many free cad programs out there, and free starting out is good. Also find all the info on g coding you can and get a firm grasp on it. While many machines can run right from cam files. Its extremly helpful to be able to read what is going on line by line as you can afjust parameters on the fly this way and save alot of time. Ensure you have the mathematics to back up calculations, algebra, some trig, def geometry, even low level calc. Once uour brain is filled with the basic knowledge only then would i move into buying a machine and start making anything. Breaking tooling or machine components gets costly, and the learning curve can be a steep one. Firearms and aerospace are some tight tolerance 0 forgiveness industries. You must understand the material properties and what is going on if you truley wish to pursue this in any fasion. It can be done it just takes time and effort.