defines any rifle with a detachable magazine as an “assault weapon” which must be registered.
I don't think that is correct at all.
So-called "featureless" rifles are still permitted to use detachable magazines. To be "featureless" a rifle must not have a pistol grip, a flash suppressor, a folding stock, a forward pistol grip, and maybe a couple more features that don't come to mind right now.
If you replace the pistol grip with a Monsterman Grip, remove the flash hider, and do without a collapsible or folding stock, then it is NOT an "assault weapon", a term invented to deprive people of their right to keep and bear some arms.