A reporter for the BBC interviewed a lawful weapons salesman, I think in Nevada. He demonstrated the procedure for selling a firearm, which was to run the person's details through an FBI database. It came up clean for the reporter (as it would have for the massacre suspect, who had never been a person of interest in any investigation). The salesman asked a few questions about the purpose for him wanting a weapon, and whether he'd used one before. On the reporter answering no, but he wanted one for self defence, he recommended a small hand gun. There were also a wide variety of other weapons displayed in the store, including heavy military grade multiple fire guns. The reporter asked why would any civilian need such a weapon. I can't quote the salesman's exact words - but this is, I believe, a close summary:
---because the government have those weapons
---if they have them, and we don't, they'd be able to overthrow us
---I hope it never comes to that, but if it does, we have to be able to defend our rights
---they are trying to demonize gun stores and gun owners to make that impossible.
I can't say how widespread that view might be, but there is powerful historic and symbolic imagery around the ideological concept of an armed citizenry as a check on government. It probably explains why some members of the National Rifle Association go way beyond defending sporting rifles, and put pressure on elected representatives to oppose any significant extension of restrictions. As election and nomination studies show that endorsement, or hostility, from the NRA can be very effective in swinging votes in marginal seats, this has contributed to the reluctance of Congress to take firm action on this issue.