The film is supposed to take place along the Nueces Strip, the coastal line from Nueces County to Brownsville. However, the film frequently shows hills and mountains, which are not found within 200 miles of the Nueces Strip.
In the scenes on crossing the Rio Grande you can clearly see the water flowing from left to right looking from Texas to Mexico. The river, of course, flowing from west to east all along the Texan/Mexican border should be seen flowing from right to left.
Cpt. McNelly's trick near the beginning of the movie, pretending he's out of cartridges is impossible on at least two levels. First, you have to pull the hammer back to first-cock and open the loading gate to spin the barrel. He spins it with the hammer down and the gate shut. Which means someone actually, deliberately, made a gun that would do what no 19th century revolver ever did, presumably because they thought it looked cool. But besides that, it couldn't work. He had already gotten off a few shots. After randomly spinning the barrel like that, you wouldn't know if there was a cartridge ready to move into place when you cocked it. Which we also don't see him do. If he had done so before the bandito pops up, he'd give the trick away, and wait, and he'd have been shot. The "trick" works on no level whatsoever.