It is actually a shotgun called a Darne, with an unusual sliding breech action. It was huge, an 8 gauge (the smaller the number, the bigger the gun). We had dummy cartridges custom made from solid brass rod stock, with the bullet held on via a screw.
Update: Mar 2021. I thought I had corrected this answer years ago, but apparently not. The gun is an 8 gauge shotgun made by William and Moore, not Darne. The info about our dummy cartridges is correct.
The Internet Movie Firearms Data Base has much more info on the guns in Tremors.
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