[Over Two Meters Tall!]

On one hand you have the incredible lethality of the weapon, and on the other the incredible durability of a Primarch.

It sounds like it does come down to the irresistable force vs. the unmoveable object as far as arguments are concerned. Unless we get a story of the Khan being found and fought in the Webway by a Harlequin, who just so happens to use the Kiss, we'll just be speculating till the cows come home.

On a more practical basis, I agree with you that if one of these things did get loose inside a Primarch, then he's toast. All the sterner stuff and not knowing their physiology stuff is just BS. Uber 40K weapons can kill a Primarch, and this certainly ranks in that category. Perhaps if a Harlequin masked the Kiss as a piece of toiletpaper, boy wouldn't that be a surprise!