Think of like this, your average pistol weighs just over a pound (.45kg) and your average rifle around 8 pounds (3.62kg)

So somewhere between 400 million and 3.2 billion pounds of metal, wood, and plastic. Between 180 million and 1.448 billion kg.

Low number is assuming all pistols, high number is assuming all rifles, so the real number is going to be between the two.

As a point of comparison, the US generates 268 million tons of garbage every year:

https://www.dumpsters.com/blog/us-trash-production

You’re talking many times that JUST for guns.