I'm not a lawyer or legal expert.This is my understanding,which could be wrong.
In the USA,there are some restricted "firearms" as defined by Federal law that are restricted .Full auto for example.
Filling out the proper paperwork and paying for the tax stamp,in some cases,permits you to own some of these weapons,but manufacturing them,even just one,is banned.Its not OK to make a full auto Sten gun.Or even a similar open bolt semi-auto.
Now,in the latter part of the Obama administration,a document was signed by the President (IIRC) that changed ITAR regulations.It somehow lumped the basic gunsmith in with exporters of weapons ,with expensive permit requirements and extensive record keeping.
I'm not sure how much that impacts the home hobby machinist. Considering this regulation,I'm not sure what I thought I knew about being able to make a gun is still true.
The way it was,if I have it right,I could make one prototype of any sort of firearm that would typically be lawful to buy or possess.
If I had the skills and tools,making something like a 1911 or a bolt action 50 cal would be lawful.I don't think I could sell it,unless maybe if I registered a serial number .I'm not real sure about that.
To make more than one,it gets more complicated.There was a form of license that allowed custom gunsmiths to make a limited number of firearms a year.Example might be a custom Sharps or Ballard rifle.I don't know if its still allowed or if the quantity was five or fifty,
To make more requires a manufacturers license.
And,all sporting goods,including guns and ammo,has a tax of ...I think its 15% ,that must be paid to the federal government.And,you don't want to forget to pay.Pittman-Robertson tax,maybe?
So,the short answer,to the best of my knowledge,the hobbyist can make one copy of an otherwise lawful firearm that (I think) cannot be sold....but the ITAR regulation makes that not so clear to me.
The last I heard,for a professional gunsmith to perform a machining operation that improved a rifle required ITAR compliance.This might be drilling and tapping for sights,threading a barrel for a muzzle device,or threading and chambering a barrel.
Its a vague,muddy regulation no one is very clear on.Those are the kind that can bite you.
Oliver: Very nice looking work!! I'm impressed.
Muzzle loaders are lawful to make,if you can lawfully own one.