As Woltz points out, Michael most certainly would have touched other parts of the gun. Of course, prints examination by law enforcement of a homicide weapon would have been routine. However, those prints would probably have been smudged by Michael's placement of the gun in his pants and his pulling the gun out.

I thought it was curious though that when Michael pulled the gun from the back of the tank there was no tape on it. Most certainly, there would have to be. While Michael's pull on the gun would have released it from the tank, it would not have released it from the tape that was holding the gun in place.