For an upcoming production we have an air rifle that is fired on stage.
The designer would like the air rifle to use a MIDI trigger of some sort that would fire the sound effect when the gun is fired. To make things more complicated, the trigger must of course be wireless. The MIDI trigger would
fire a sampler. Does anyone have any experience with or is aware of an "off-the-shelf" device that would perform said function?

The 'Interactive Fx Controller' can be organized to work using a standard radio mic transmitter and receiver.
Essentially, it switches a tone generator (which is about 1cm square), which sends a complex telephone tone via the transmitter that is obviously picked up by the receiver, and the 'on' and 'off' of this tone triggers MIDI events programmed in the fx controller.
Note that both the on AND off trigger separate events.
I have used this with a Thompson sub Machine gun (easy to do as there is a usefully large magazine to hide the Radio transmitter and the tone generator), so the pull of the trigger 'fires' a looped machine gun sample, and the release of the trigger not only stops this sample, but also triggers a 'clunk' sound to simulate the firing pin returning to base. All quite effective sound-wise - you need
some smoke etc. for the visuals.