#1 The 2D Amendment says "shall not be infringed." -That includes machineguns; weapons which should've never been made difficult for the general public to own.
#2 It is not a functioning automatic weapon. It is an attachment that causes multiple activations of the trigger group in quick succession - still semiautomatic fire, equating to one round fired per pull of the trigger. Albeit that fire is quicker than with your finger. In order to for it to be a machinegun, or fully automatic weapon it would have to fire multiple continuous rounds with each pull of the trigger. Even a select fire (3rd burst) M16A2 and later variant/M4 is not a machinegun - nor is it full auto.
#3 yielding language to the adversary and those whom wish to infringe on our rights is what screws us over. That holds true even if it is over items with no practical purpose other than amusement. Adding a bumpfire/slidefire stock to a semiauto rifle doesn't make a machinegun/automatic rifle anymore than starting a car and dropping a brick on the gas pedal makes a self-driving car with autopilot.
I agree with everything you said...which will get you nowhere with the American public at large. You make that speech to 100 people who are not gun owners and you might as well be talking to a wall. All they see is a rifle that fires at a very high rate; an automatic weapon.