After a little thought, it becomes apparent that Sex Machine's crotch gun is a terrible idea. Now, ignoring the fact that the cylinders cannot feed the gun the way they are positioned, it looks to be a pretty large caliber - if you were to fire that thing, you would pretty much be punching yourself in the groin.
Maybe Sex Machine is into that sort of thing?
In an interview with James Rolfe, Tom Savini (Sex Machine) justified the recoil issue by stating that during his time in the army (he was in the Vietnam War.) they tested the recoil of an M16 by placing it on various parts of the human body including the chin and the crotch with no injury what so ever.
In the series, Sex Machine lost his dick in a smelting accident. The crotch gun is a literal penis replacement. At least until it grows back after becoming a culebra.
It could be belt fed, one cylinder holds bullets and the other holds the spent shells, it resembling a revolver is just a cosmetic look, also, the codpiece itself might be reinforced and padded to reduce the recoil, seeing as they would need room for the firing pin and the trigger mechanism (which the same film had shown it was fired by a hand pump)
The conquistadors arrive in the vicinity of the temple during the reign of Queen Isabella of Castile, but she died in 1504, over a decade before the Spaniards began exploring the North American mainland. At first glance it looks like a plot hole, until one realizes theres a very good reason why that whole expedition would vanish from history.
Professor Tanner can be quite The Load in the series and no where is that more clear than in the episode where they head into the temple itself, where he accidentally drops all of the weapons into a machine, crushing them and making them useless. Quite the klutz... Or maybe not. Professor Tanner was actually making sure the group was defenseless against the Culebras!
Earl's general apathy towards Christianity is given more weight in season 2 where it's revealed that he ran into one of the Lords of the Night many years ago.
The fact that the map to the Blood Well is in the back of a bunch of oil bonds seems strange, until it is later shown that the location of the well was discovered by an Prospector in 1912 who was digging an oil well, who was possessed by the Santa Sangre. The possessed Prospector may have been active in the world long enough to try to make sure other humans would find the well. And what better way than to put the map into something as valuable and bonds?
The series establishes early on that all it takes to kill the Culebras is to destroy their hearts. So why do bullets never ever work? A bullet to the heart is pretty effective at destroying it.
The heart is, in essence, a set of extremely strong muscles, unlike our other organs which are made up of softer tissues, so it probably depends on the gun and your aim, put one hole in it, with a regular 9mm pistol, Culebras probably heal that up, use a sub or burst fire gun with the same 9mm rounds, it will tear a heart to shreds with proper grouping, use bigger guns like a shotgun and yeah, they probably won't come back from point blank.
"Protect and Serve" establishes that Xibalba has a rigidly enforced social hierarchy with Amaru on top, but also that Amaru has been trapped in the blood well for almost as long as the Nine Lords have been on Earth. Who has been running the show on Xibalba all this time?
Probably whoever was next in line in Xibalba while the top dogs like Amaru and Brasa were imprisoned, if the hierarchy is strictly enforced, Amaru would have been put right back in charge the second she got back.
The Nine Lords are the ones who escaped Xibalba and defeated Amaru and her court, so how is it that the Lords apparently have no idea that the "treasure vault" they are searching for contains her blood and soul? After all the Titty Twister was also pulling triple duty as Kisa's prison/Altar, The Labyrinth and a Prison for the rest of the Xibalba Demons they captured.