Battle Rifle kill in Swatnums

My friend got a gun punch while playing Swatnums:

https://www.halowaypoint.com/en-us/games/halo-5-guardians/xbox-one/screenshots?page=1&gamertag=radar3301enigma#ugc_halo-5-guardians_xbox-one_screenshot_radar3301enigma_986eb6ef-3fed-48d2-97fa-e878943d3472

What’s up with that?