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"and when the spotter, briefly stunned by whatever unspeakable Warp-spawned horror he had seen, slowly dropped his binoculars, sergeant Graver saw that he could see no more, for bearing witness to the sheer fact of the Slaaneshi abomination's eldritch existence had burned the man's eyes out of his skull. blood oozed out of the empty pits in his face as his mouth dropped in a horrible scream of despair."

Of course, you neglect to mention that the trooper drops the shell out of shock at what happened to his friend, which causes it to detonate, leaving nothing left but a wrecked tank and a bloody shoe.

Even simply spotting targets from behind the lines is dangerous when the universe is as grimdark as grimdark can be.

an artillery shell probably won't explode from being dropped, unless it's some "smart" shell with a very cranky machine spirit

GRIMDARK is the only explanation you need.

Andy the Earthshaker round.

Demonette perforimng french cancan.