The update now allows certain cases to drop when completing contracts. The case drop system works VERY similar to Counter Strike: Global Offensive, you have a chance at obtaining a variety of new weapon skins. Skins that can come in 3 conditions, and have a rare chance at also coming with a stat trak(you'll have a little number on your gun showing your kills, it's basically a strangifier)
As for the UNUSUAL effects, these are the rarest of qualities. These modify your gun to have a particle effect applied to them, much like VALVE quality weapons. You have Isotope(a green hypnotic type of ring effect), COOL(Turns your gun ice/navy blue with some snow flakes falling and having a icy mist to it), HOT(which makes your gun RED HOT with a steam effect) and then Energy Orb(which makes your gun red and cloudy like the energy hole effects) - As of right now, none have been found, but my guess would be ANY weapon can have the effects on them. Much like UNUSUAL hats, there's a variety of effects for most hats, it wouldn't make sense to have them on stock only.