I can expect that firing straight up into the air, my shot will only reach about 30 meters (or yards). Maybe 40.
Umm, no. Your shot will reach considerably higher than 30 feet straight up.
Muzzle speed from a 12ga is on the order of 1500 fps (call it 450m/s for the metric types). For it to have a ceiling of 30 feet (10 meters) straight up, the shot would have to be decelerating at ~50G's.
And that isn't happening unless you're shooting through a wall/ceiling.
Note that the problem with taking things out when shooting straight up is that human's are really pretty crappy at judging distance directly overhead. Which makes judging the lead you give the target pretty much guesswork...