You mean "assault weapon". The laws invent the category of "assault weapon" and then define it arbitrarily to ban it. "assault rifle" is something completely different and actually has a definition, but is also something that's illegal for sale anywhere in the US (if newly produced). Assault rifles are defined by having selective fire between semi-auto and fully-automatic. (These weapons are still sold in the US, but only old weapons grandfathered in, and require special permitting to own.)
You make it sound like the guns are using loopholes, but it's more that a lot of these bans are against a bunch of random miscellaneous attributes of the gun and don't make sense to begin with.
For more see:
https://en.wikipedia.org/wiki/Assault_rifle
https://en.wikipedia.org/wiki/Assault_weapon