Read up on Jeremy Kettler's attempt to avoid interstate commerce and also Wickard v Filburn. You probably know about the latter but not the former.

If growing your own plants with nothing but seed and material from the earth on your property, and then feeding it to your local animals is interstate commerce.... then it's hard to imagine any firearm even if made of iron pulled from the earth underneath you and machined on site and never sold to anyone or hell even fired could be considered to not have interstate nexus.