Grats, but if you're going to try and help people, it usually helps to make what you're saying feasible.
You claim that you had 5 blue tiger guns by level 20, when in theory, it's next to impossible. Lets say you did TDM, (which you most likely did, and if not, probably search). That's 10 points per kill, correct? To get blue tiger, you need 75 headshots with that particular gun. We know that nobody gets 75/75 headshots, and it is more than likley that you will reach marksman III before expert II. Lets go by the off chance that you do get 75/75 headshots, thats 750 points alone in kills. You have to add in the 350 for marksman challenges, and 350 for expert challenges. By the time you reach blue tiger, thats about 1500 experience, assuming you're a god and always get a headshot. Otherwise, you'll probably have about 4000 exp when you reach blue tiger...headshots are about 1 in 4 for me, and I'm good.
Now lets assume you are a god, and you have 1500 exp with your one blue tiger gun. You do that 4 more times, and you have 6000 experience. We all know that you unlock challenges at lv. 5, and that takes about 3000 total exp to reach. That's 9000 experience-- minimum. Now in reality, you'll do about the 1 in 4 to 5 headshots, which in return gives us 280 - 350 kills before blue tiger with a single gun. That is, say, 3000 - 4000 experience per gun. 4 * 5 is 20000 exp. Now throw some challenges in there, which you will get no matter what, and then you have those match bonuses. You need about 20,000 exp to reach lv. 20.
All in all, I'm extremely bored, and you won't have 5 blue tigers by the time you reach lv. 20. Oh, and its 1000 exp for reaching red tiger...not 2000. >_>