So guns aren't assault weapons when not being used to assault people but when they are they're still not assault weapons just misunderstood pecadilloes.
Come on, this isn't that difficult to understand if you just put in some effort to try.
If someone buys an AR 15, or similar automatic rifle, for the sole purpose of shooting targets on their back 40, then it isn't an assault weapon. It's a toy.
On the other hand, if someone buys one of these guns with the intention of shooting people then it's definitely an assault weapon.
If you aren't clear why the intent of the owner makes a difference, then you need to understand that the term "assault weapon" is NOT a description of any physical feature of the gun itself - it's entirety a description of how it's intended to be used.
Incidentally, if the owner of an AR 15, who is using it solely for recreation, sells it to someone who plans to use it to go on a rampage and shoot up a Starbucks, then the gun becomes an assault weapon when it changes hands.