The gun is going to come down somewhere. Gravity is a bitch like that. So, coastline, and make sure the winds are out to sea. Not just the surface winds, but also the winds aloft. When the gun does eventually come down, it is still going to be connected to the balloons, and will be floating at the surface for days. A balloon mass this large could attract attention. You'll want some kind of water soluble string to drop the gun and ensure it sinks.

As it ascends, the balloon cluster is going to show up on doorbell cameras. The noise of the gunshot is going to draw witnesses with phone cameras. At the very least, you need a moonless night. Fog bank and low ceilings can help. Definitely don't want to try this during the day.