They are 100% percent random giving you a supply drop every 3-4 games on average, it does not depend on what gun etc, bu I had been using the mors pummler for 5/6 game and I didn't get supply drop and once I started changing my guna around I was getting supply drops more often, they say you get a supply drop every 45 mins by I see tha untrue or inaccurate but myself personally if you used a different range of guna perks equipment, you are rewarded more often but I am also unsure of this it just happened to me yesterday!