Visible crew/equipment: When the Shriekers are trying to find Burt in the front loader (tractor with bucket) with their tongues, as the camera pans it catches the arm of a puppeteer controlling the tongue, in front of the front loader. (01:18:00)
Continuity mistake: When Burt is hiding in the bulldozer scoop, Grady looks down at him from the top of the oil tank and we see that Burt has his knife in his hand. However, in the next shot, we see him pull the knife out of its sheath on his leg.
Visible crew/equipment: When Burt shoots at the Shrieker with the high-powered rifle, a frame of two before the Shrieker explodes, a wire is visible at the tip of its beak, holding it up. (01:16:45)
Answer: It's a 50 caliber Grizzly Big Bore rifle. They are real. Just put "Grizzly .50 Big Bore" into your search engine and you will find plenty of sites.
Grumpy Scot