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 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.
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.
Continuity mistake: At the end where the building explodes, everyone stands up from the pit. Burt is standing up in one shot, but in the next, he is kneeling down.
Continuity mistake: The scene where Fred is trying to rope the ostrich he gets pulled for a couple of feet and looks up at the bird. If you notice his shirt is dirty from being pulled by the bird. When he gets up to meet the taxi his shirt is magically clean.
Continuity mistake: When Earl enters the barn after being hosed down with CO2, the amount of vapor as he warms up varies from shot to shot with it almost not appearing to it heaving pouring off his clothes.
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: While driving in search of the graboids, the blue square blip on the seismometer represents their truck, and it's moving. However, when they are using the RC truck, the seismometer shows the blue blip is still moving, but the truck is now immobile.
Continuity mistake: Earl and Kate are talking about dating at the end and walking with Burt and Grady towards the hole in the ground. In the first shot, it appears to be overcast. Suddenly the sun appears on everyone.
Continuity mistake: When Grady and Earl wreck their truck, Earl picks up the radio mic to call anyone at their base site. He is talking into the back of the mic in two shots, then correctly in the next shot.
Continuity mistake: When Earl and Kate are talking (about her Playboy photo) while stuck on the roof, she is wearing her ring on her left hand (it was on her right earlier in the film). In the film's timeline, it is minutes later that the ring disappears.
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)
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: 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)
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