It is rather more difficult to make a gun than people imagine, getting it to go bang is one thing, making sure the result of the bang is a bullet leaving the gun at high velocity and not several large pieces of metal imbedding themselves in the firer's body is another, theres no challenge in finding the right material for making it, just look up some of the tougher steels, but you need skills to machine that metal into the right sized bits and then make sure they fit together and work, that takes practice.
As for 'grinding the weld off the barrel' type reactivations.... once I've done that for you, feel free to fire the gun( once I'm safely away from you and ideally behind concrete walls ), the only way that gun could be reactivated is to put a new barrel on it (see above paragraph for howto make a new barrel)
anything else is just fantasy cooked up by the media about guns (along with my favourite "Making a plastic airsoft gun into a real firearm by putting a metal barrel into it" )
Finally, anyone with the skills and knowledge to be a gun smith can make far more money working as a gun smith rather than a dodgey bloke in a shed thinking 10 mins with a black and decker drill can make him a skilled man.
But you all dream on