I suspect they were being shot with too small a calibre ie 223 , if shot with a higher calibre 3006 308 270 and upper head shot they will die . If you miss the head then you end up getting the neck , and they will die from a neck shot .
When you shoot cattle, deer , centre of mass you can get a gut or lung shot which can cause the animal to live for days wounded , and therefore center mass shots should be avoided .
I am guessing it is difficult to delineate a heart shot from a helicoptor but a head shot should be relatively easy if you miss the bullet just goes into the ground not into the lungs or gut as with a center mass shot .
I have culled cattle I am guessing similar skull thickness as camels , death is instant , did have one guy Ponti , come buy a beast off me for a tungi , he shot the cow in the jaw , instead of upper head , was a disaster , cow ended up jumping a fence into the neighbour's who had to put it out if its misery as blood flowed out its jaw .
Really depends on the skill of the person and calibre of the weapon , but mass cullings that are humane should not be an oxymoron .
Alternatively corral the camels in temporary corrals , then truck them to an abattoir where they can be killed humanely with a bolt to the brain .
