Maybe The Largest supply -by far- of various surplus US military rifles etc is very tightly locked away in various buildings at a Naval facility.
There is no possible access by any commercial firms
or the CMP. I won't name the state, but a few of you might know where it is.
As to how much they have of which types, my contact was not told, but the huge quantities could stretch back to WW1. He chatted in person with a guy who was on active duty there about two-three years ago. The guy indicated that the vast numbers are beyond what anybody could ever imagine.
The previous commander wanted to delay his retirement, being concerned that after he retires, a certain "element" of our civilian govt. might somehow learn about, and contemplate doing something really bad and stupid with the surplus "supplies".