The plane pulls up sharply from a steep dive and right when everyone and everything inside would be experiencing maximum downward force, they use it as a device to do the exact opposite and have his gun float up into the air.
Stretching physics in a movie is part of the game. Happens in almost every movie and repeatedly in most action movies.
But making it work exactly opposite of reality and also make it facilitate a climax is something else.
Edit: my bad. This entire criticism was about Non Stop with Liam Neeson.
Carry on 😉