Question: Whilst atop the oil towers, Grady asks Burt if he's sure he doesn't have any more bullets. When they blow the Shrieker off the top of the blue Volvo, it's evident they have quite a few bullets left for the elephant guns. I get that they're slow at reloading and they'd be dinner if they tried to take them all on at once. But why not have someone sneak into the office, grab the guns, and go back to the oil towers, then pick them off one by one?
Answer: The elephant gun was used in the original Tremors. When Burt returned in the damaged truck, he said he was completely out of ammo.
Answer: The elephant guns weren't in the office; they were in the back of Burt's truck, where they placed them right before they lifted the captured monster. Given that the truck was in the garage and the garage was the location from where the majority of the monsters came, it was unlikely they'd have been able to get to the guns.