1. The pot of "boiling water" was obviously cold, with dry ice making it bubble. Really fake.
2. Mandingo falls backward from some kind of recoil that didn't affect Hammond Maxwell who fired the rifle. That's pretty typical Hollywood though.
3. Hammond Maxwell shoots Mandingo with a SINGLE SHOT RIFLE. Then Agamemnon shoots Warren Maxwell with the SAME RIFLE which has NOT BEEN RELOADED. That huge gap in logic totally ruins the ending. Lazy writers/producers.
The recoil effect may have been pure Hollywood, but the rifle was a revolving rifle, an early form of repeater. As the name suggests, it was a rifle with a revolver cylinder and action. Colt was probably the best known brand, but Hammond's appears to be a Le Mat. The Le Mat carbine was based on the Le Mat pistol, which was known for combining the revolver action with a single shot shotgun in an over and under barrel arrangement, with the cylinder essentially revolving around the center of the 'under' barrel. The cylinder can be most clearly seen when Agamemnon is pointing the gun at Warren; in other shots the barrel rings and ramrod can be seen.
If it was a Le Mat carbine, it was still a mistake if the story is set in the 1830s and that weapon, capable of firing nine or ten shots, was not invented until 1856. http://revivaler.com/lemat-centerfire-carbine/ If it was some other kind of double-barreled rifle, still a mistake as Hammond shot Mandingo/Mede TWICE--once in the upper arm or shoulder and then with a shot centered enough to throw him into the boiling pot. So there should have been no shots left assuming there were only two to begin with.
I like your first point! Seems you wanted to see the actor really being cooked. Well, it is the "art of moviemaking" that what´s on the screen is not really happening. :-)