Neither.Not a ten gauge, a double rifle.
From imfdb.org:
William Moore & Co. 8 gauge shotgun
Burt Gummer (Michael Gross) owns an "Elephant Gun" that is a Belgian-made William Moore & Co. 8-gauge firing solid slugs. The shotgun was provided by Ellis Mercantile for the Tremors film. When it was thrown from the truck to the rock, it was dropped and the stock shattered at the wrist due to the 12.5 pounds that the shotgun weighs.