Illegal by federal law I do believe. What do you think? Would you call this crime extremely under-enforced?
There should be NO exceptions to the constitution other than those explicitly written into the constitution. From the text of the second amendment: "the right of the people to keep and bear Arms, shall not be infringed". Much like the Ten Commandments, the constitution and amendments stand as ultimate law, no additional conditions or exceptions, no matter what do-gooder tries to add their own spin.
