Continuity mistake: When Kate moves the tray of core samples from the centre table, she puts it down on top of one other tray. When the camera pulls away, and for the rest of the scene, there are two trays under it. (00:16:15)
Continuity mistake: In the scene where the Graboid is pulling the truck you can see that there is a gun rack and a gun in it. Right before they are about to hit the rock you can see that the gun falls off of the gun rack. The gun goes on and off the rack between the following shots. (00:30:25)
Continuity mistake: When the Graboid scares Earl and Grady into backing up the truck over the ridge and getting it stuck, the hill they stop on is empty - that is, no rocks. But later, there's a big rock right next to its right side that Grady jumps onto. Also; when the truck first crashes, the angle makes the windshield roughly level with the ground. But subsequent scenes show the truck at a different angle. (00:42:20 - 00:47:35)
Continuity mistake: When Earl and Grady arrive at the radio tower and see that it's been all torn up, Grady sits down - we can see he has an ammo sling against his chest, as it is reflecting moonlight. The camera angle changes and his canteen is now in front of the sling. (00:54:15)
Continuity mistake: When the walking Graboid takes Julio at the office, Kate tries to hide and knocks over a plastic container, which spills empty beer cans on the floor. Moments later, without having been touched, its position is a little different. (00:59:50)
Revealing mistake: When Burt returns after being ambushed by Graboids, Grady is holding a rifle across his shoulder. He has two fingers in the trigger guard where there should be a trigger. You can see it has no trigger. (01:06:29)
Plot hole: When Earl, Grady, Burt and Kate are running from the truck that Burt shot, they go towards the cantina, which has a couple of smaller blue and white structures beside it. In a wide shot as they're running, we see the 1st shed at the side of the frame. A moment later Earl sees several walking Graboids in front of the 2nd shed. The beasts were in plain sight, yet nobody noticed until they were almost on top of them. (01:14:50)
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)
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)
Factual error: When Earl swings on the light fixture and the cable detaches from the ceiling due to his weight, he falls 'a little' before hitting the wall. He would have dropped like a stone immediately, and likely would not have reached the wall. (01:28:35)
Audio problem: In the scene where the Grabboid is pulling the truck with the chain attached and the guy is trying to shoot it, the gun noises do not match up with the muzzle flashes at all. At one point it is obvious he fires 2 shots, and yet 4 "bangs" can be heard.
Revealing mistake: It's very clear they blew up a model of the buildings. The grass seen just before the explosions is way too long and is not as long when the characters are running away. Further, the truck and the barrels have disappeared at the explosion shot.
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.
Continuity mistake: In one of the first scenes when Grady and Earl are riding to the oil site, Grady attempts to take the last soda. If you watch Grady closely when Earl tries to play Rock, Scissors, Paper at first he is holding up his right hand, then it goes to Earl, then it goes back to Grady holding up his left hand.
Revealing mistake: When Grady & Earl blow up their first graboid, one of the pieces of orange goo that hits the truck flies in from the LEFT. All the other pieces of orange goo get flung in from the right.
Audio problem: There is a scene just after Earl and the new guy arrive at their base, when a truck (driven by one of the Mexicans I think) pulls up from the left hand side of the shot. If you listen you can only hear the truck's engine from when it comes into shot. This suggests that it was waiting just off shot and then drove in, rather than coming from any distance as you would have heard it approach.
Continuity mistake: Just before Earl gets sprayed with the fire extinguisher, he wraps a shirt around his hand. When he takes the shirt, you see he has no gloves on. Then when he asks for gloves the camera cuts to a different angle and the gloves are on him, even though he never even took them off the other guy.
Continuity mistake: In the scene when Grady says "son of a..." when the Graboid is dragging the truck, there is a shot of the truck driving forward.
Continuity mistake: After Burt gives the double rifle to Grady he closes the box he got the rifle from. Over the next shots the box changes from being closed to open and closed again two times as the shot changes.
Factual error: As Burt is driving, he is producing a video diary. He indicates it is 1900 hours (7PM) on May 14th, and yet it is pitch black outside. Nowhere in Mexico would the sun have set by 7pm in May.
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