If a person reloads for himself, he does not need a license. But if he does the reloading and accepts payment for the product, he is "engaging in the business of manufacturing" ammunition. I don't think it matters if the reloader sets a regular payment or accepts "courtesy gifts".
The same would be true for repairing firearms "as a hobby". If a person repairs firearms and takes money or anything of value for it, it is not a hobby; he is engaging in business as a gunsmith and needs a license.
Also, in either or both cases, if the person messes up, he can be sued and his homeowners' insurance won't cover "business" activity, especially of dubious legality. And believe me, one's "friends and buddies" will sue in a microsecond if they think there is money in it.
Now to the technicalities of chamber adapters. The analogy to a revolver is not a good one. In a revolver, the gas escape is to the open air. With a chamber adapter, the gas would escape into the gap between the adapter and the barrel and exert pressure against the adapter, trying to force it back out of the barrel. That would exert more pressure on the bolt lugs than the normal cartridge would since the adapter would be larger.
And of course, each adapter would have to be made to a its own headspace, a bit tricky with removable adapters.
I don't say it can't be done, but it requires a lot of thought, hopefully by folks smarter than I am. I will say that if it were easy, I think some company would already have put the idea on the market.