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.
Factual error: After Grady and Earl wreck their truck, Burt, Earl and Grady have a radio conversation. At one point Grady interrupts Burt's transmission cutting off his voice, but making it appear Burt can hear him. It's not possible to interrupt a transmission while the other person is also transmitting even though they're using repeaters which is why Burt's voice was cut off in mid sentence. The only way you can transmit and receive at the same time is if you're using full duplex. Burt's voice was cut off, therefore they are not using full duplex.
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)
Factual error: When Grady and Earl are trying to sleep on a boulder, there's a very large moon in the sky. It's clear that it isn't the actual moon. None of the craters or other markings correspond.
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