How about a bow and arrows? those are built as killing tools. sure you can take one to a range, but their sole purpose was crated to kill. I can't find a single state that prohibits felons from purchasing a bow and they don't require any background checks at all.
I'd argue that they should a also be regulated and restricted, especially combat style cross bows and things.
But as mentioned, I feel you have to start somewhere and that's place should be at the top where things are of greater concern at the moment.
Shouldn't deflect from that with "what about ....". People like to argue about where the line should be, and yes it's difficult, but difficult shouldn't be used as an excuse to not do anything.