Like I said, a glitch, you just emptied your magazine and glitched reloading or the next time you fire, Clever GM says the last 100 rounds you just emptied made the chamber hot and on your bad glitch it starts to cook off your ammo, whoops you missed that heads up on your HUD! Make an agility + Armorer if you have it test to see if you can pull the mag out before it empties!
Not, You just emptied 300 rounds from your caseless Fatal Light, and I'm tired of seeing you shoot, your next magazine cooks off.