So what is the purpose then if it acts exactly like a semi-automatic weapon?You are obviously uneducated on how a bumpstock works. The trigger is depressed and released for each release of the hammer, it does not continually cycle unless you depress and release the trigger each time. Atf definition listed above.
But it doesn't, does it? The rate of fire with a bump stock approaches that of a fully automatic weapon, which you can't readily do with a semi-automatic weapon.