How was Cragie's plan going to work when his prints were on the gun?

Basically Cragie shot an unarmed kid by accident, and then planted a pistol in the dead kid's hand, to make it look like police self defense.

However, there was an investigation as to whether or not the shooting was justifiable, since why didn't the investigators check to the gun for fingerprints? Or maybe they didn't do it yet, but why wasn't Cragie worried about that?

A better plan would have been for Cragie to plan the gun a few feet away from the kid and then say he himself took the gun from the dead kid's hand, in case the kid was a still alive, or still a threat.

That would be a more plausible explanation as to why his prints would be on the gun.