You need to bring up an example where "right wing" people have attacked a marathon, or civilians in general.
You can only get guns at gun shows two ways. One is you go through a dealer and if you have a concealed, or depending on the laws some other kind of license, can pick it up that day. Or you buy it from someone else who happens to be selling it with cash. Just like you can buy a car, or anything else that can be used as a weapon over craigslist. Doing it the latter way will bring up some serious legal problems if the person uses that weapon and its brought back that you sold it and you don't have some kind of paperwork saying you sold it to them legally.
I don't care how decked out your AR-15 or AK-47 is. It is not military grade. No one in the armed forces is using what you can buy at a gun store or online. They all have select fire weapons or fully automatic weapons with grenade launchers and other things that ARE NOT legal to own in America since the 20s without a CLASS 3 License. The guns used in the mass shootings are not military grade. Honestly if they were using shotguns alot more people would have been killed, and no one is after those.
Families that own automatic weapons have gone through several types of paperwork to get a CLASS 3 and have paid several hundreds, to several thousands of dollars to own FULLY AUTOMATIC or SELECT FIRE weapons and have to go through several background checks and alot of time are gun store owners or just rich people that are not going to be using them in mass shootings. No mass shootings have been using FULLY AUTOMATIC or SELECT FIRE weapons other then cartel style things or like old gangsters with Tommy guns or Bonnie and clyde, etc. Body armor is not legal in alot of places and even if it is it costs alot of money.
Also that church is crazy. Let them picket. If you are going to let Neo Nazis picket things, you got to let them picket. Its there right to do so as long as its non violent. If you take that away you might as well stop non violent protests for everything and turn America into a non free country.
And as a reminder this person used a bomb, not a gun, to do this.