It would have been corn cob media. What always bothered me is that she just fired the big Dillon tumbler on without tightening the lid on. I tried that with my CV-500 to see how much noise that would make. Pretty deafening. No way would that have been left like that if you were to stand there. They would have had to do some ADR looping for that scene.
That said, yes Tremors is a high water mark in shooting ####. My Attack Pack gets a regular watching. Burt/Hiram Gummer is oft quoted. "I feel I was DENIED critical, need-to-know INformation." or "I feel I have not been privy to critcal, most needful information." My favourite of all 4 movies has to be the 4th.