Here is a simple argument to support that stance. Under the 2nd amendment I have the constitutional right to bear arms. If I want to purchase a suppressor to limit hearing damage while firing my weapons I have to pay a $200 tax, send my fingerprints to the FBI and wait 6 months to a year and sometimes longer before I can take possession of my suppressor. There are no similar restrictions on the right to vote or speak freely. The same rules apply if I wanted to add a stock to my AR 'pistol' which currently has a pistol brace or if I wanted to add a small piece of metal to make it fully automatic.