Damn, RNG, Chill! O.O

You get 5 card slots in an AR pack, 3 uncommon and 2 rare. The first uncommon slot is always a gel etc until you max them at 255, then that slot is an uncommon equipment like an AR III or Disruptor III etc.

The 2 rare cards have a higher probability to give you a rare gun card until you max those out (they can drop rare character cards unless you have unlocked them all or a lvl IV equipment card). Each rare slot also have a 7,5% probability to drop an UR card which remains the same until you have them all.

So the best way to max rare guns is to buy arsenal packs, if you want UR cards all 3 99k packs have exactly the same probability.