[MakarovJAC]

I would like to know how's is the deal when, lets say, developing a gun. I know handcrafted firearms are banned in many countries of the world. But if I wanted to desing, and build a gun, I'm forced to become a licensed gunsmisth (or an engineer); or I've to work with a company that manufactures weapons?

[BillM]

Kind of depends on where you are. In the USA, if there are
no state or local laws forbidding it and it doesn't fall under
the arms controlled by the 1934 NFA, go for it. Even some
of the NFA items can be "homebuilt"---SBR, suppressor,
SBS, maybe even AOW. You just have to play by the rules and
have your paperwork in order BEFORE you build.

Want to design/built full auto? That takes a license. Want
to build guns for profit? License. For your own use or to be
the mad inventor toiling in his garage on conventional
firearms? Go for it as far as I know.

Keep in mind that the above statement is probably legally
worth exactly what you paid for it.

[c3shooter]

You must be licensed as a "Manufacturer" FFL, only IF (a) weapon in question is a National Firearms Act weapon, or (b) it will "enter the stream of commerce". The BATFE regulates firearms that are transferred to others. If you want to get a REALLY sharp pocket knife, and a chunk of steel, as far a Federal law in the US, you are welcome to whittle yourself a 1911A1. FOR yourself. Or get a chunk of plastic, and make yourself a Glock.

Now, if you want to make them to SELL- or to give to another- get the license, and an accountant- you will have Federal Excise taxes to pay. Unpaid taxes put Ithaca out of business.