That we are all a bunch of uneducated rednecks and goobers. That they hope we die of self-inflicted gunshots, our child(ren) kill themselves (with our firearms), kill our child(ren) (AD, or otherwise, with our firearm), or otherwise suffer from a firearm-related fatality, either ourselves or a family member.
As to the musket comment, that shows their ignorance of both the available firearm technology then, and the understanding of our inalienable rights by our Founding Fathers. In France, in the 1600s, a Frenchman invented a musket that could fire 16 balls consecutively (aka, automatic). There was the Puckle gun, invented in 1718. A large gun on a tripod with a 9-round cylinder. It was designed for naval use, with a 32 mm bore. Circa 1780, Girardoni (Girandoni), invented an air rifle used by the Austrian military for about 3 decades. One of those accompanied Lewis and Clark on their expedition, which they credited as keeping them safe from the Indians they met along the way. They would display the remarkable ability of the rifle to rapidly fire .46 caliber lead balls. It had a tube magazine with a 20-round capacity (I have also read 21 or 22 balls). Those are some of the more known firearms, one of which was acquired by Jefferson. So, obviously, they knew about rapid fire arms well-before the 2A was written. Aside from that, the 2A states “arms”, not firearms. Our privateers that had commissions to attack British shipping did not use spit balls and sling shots, they had cannons and other arms. Many of the arms used by our militia men and Continental Army were purchased by wealthy colonists, as the revolutionary government had little money. So, clearly, ownership of arms by private citizens was not an issue. Even today we are “allowed” to own Muzzleloading cannons manufactured in or before 1898 (and replicas thereof) that are not capable of firing fixed ammunition. The ATF does not list them as destructive devices requiring NFA approval.