Carl Howey
Great idea, what could possibly go wrong.

SpiritDrinks
Is your tank full of pure CO2 or Nitrogen only? If not, you will end up with skunk beer. The oxygen will cause bad things to happen to your beer. Bacteria love the stuff.

Taxarod
Hello, Apart from the bacteria problem which I don't have a lot of knowledge about.. You really need to check what's the output presure of the tank, most tanks output either 450psi, 650psi or 850psi, unless you have a very specific SLP ninja regulator (around 250psi output) this would be quite dangerous.. I'm quiet sure that a keg won't take a lot of presure. I don't think 300 psi could be your output, more than likely its your operaing presure of your gun.

Jolly Imbiber
Great idea, no no not stupid at all. Be sure to regulate the pressure.