Baseball bat, knife, sword, a small amount of rope, axe, hatchet, machete, chainsaw, fireworks, gasoline…
Here’s the thing:
You already have to pass a background check.
So what more do you want? After that the criteria start to become subjective and will be applied be racists to disarm minorities and poor people.
Now, I’m actually for some more generalized gun laws, like requiring that the gun or ammo be behind a lock when you’re not in control of it, but that’s not really relevant to stopping mass shootings. Ending mass shootings (a very small fraction of gun deaths) is way more about ending the desire to do such a thing.
We’ve had easy access to guns for a long time, but mass shootings only started in the 90s, when angry white men felt they were getting left behind and had no way to feel valuable in the new society we’ve been working to build. I would suggest this episode of Some More News to get a quick understanding of angry men, and the book Angry White Men by Michael Kimmel to get a much deeper look at who these people are and why they act and feel the way that they do.