Nice scratch builds! Love the paint jobs too, great colors.
One observation & suggestion: They look more like Kannons or Killkannon's to me than
KMK. (ref:
http://wh40k.lexicanum.com/wiki/Ork_Artillery#Killkannon, http://wh40k.lexicanum.com/wiki/Ork_Artillery#Kustom_Mega-Kannon).
KMK's are really the Ork equivalent of a plasma cannon. You might consider adding a few gubbins to the models that give the sense of unstable electricity flowing through them... maybe some extra wires? If you could buy or scavenge some beads, plastic balls/spheres, or LED's and paint them a plasma-like color (blue, red, purple, etc.) that might do the trick. Just a suggestion!
I think they're perfectly fine for count-as in friendly games. Check with your opponent first of course. I would have no problems playing against them!
Thanks for sharing!