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Games Workshop did make a ruling to that effect at one point (the same time they made a ruling about bio-plasma being a rolling effect that could pass through other units to get to the target), but were very clear in pointing out that the Carnifex is effectively a wall which other creatures can hide behind. Whilst you could single out a model near to the Carnifex, you can't 'ignore' the Carnifex to shoot a model behind it if the line of fire crosses the Carnifex's base. There was some argument amongst the Rulz Boyz as to whether smaller creatures were protected by this ruling as they could largely be seen between the Carnie's legs and such, but to my knowledge, anything behind the Carnifex is not open to attack.

As another ruling in favor of the Carnifex, and all of the big creatures, was made allowing them to 'walk fire' through a squad in close combat. In effect this meant it did not matter how many models of a squad the monstrous creature was in base contact with, the monstrous creature would remove as many models from the squad as it caused wounds. This ruling was made as an Imperial Guard player in the Studio discovered a very canny loophole in the rules which let him string a squad out with 2" between each model and his enemy's 'fex and Tyrant would spend the entire game killing a whopping 3 or 4 models each since they had to charge the closest enemy model if they were going to charge.