DoD approved crosscut shredder for classified material and as I recall I think it was 7/32 of an inch but they changed that to millimeters now and here’s a link to several varieties available.
https://www.papershredders.com/DoD-High-Security_c_36.html
I don’t know if anyone recalls this but in the movie The Falcon and the Snowman there’s a scene where they’re in a classified space and they’re using the shredder to mix their Bloody Mary’s I always kind of got to laugh at that because there was special Shredder oil you had to use that was a very fine oil and I can’t imagine a bloody mary tasting very good after it’s been through an oily shredder.
After Fawn Hall and Ollie North were charged with shredding documents I recall most classified shredders in the military were stenciled with the name Ollie prominently displayed. So Ollie’s legacy in the military was to have practically every Shredder named after him.
In a high output SCIF you use a pulverizer. You can throw in a something like a NY phonebook into the invisible blades and it vanishes leaving just a powder. Tapes feed themselves in automatically once you get them started.