Well there are 200 Capsules. In those 200 capsules, there is 1 gun placed in them. You should buy them, whenever they are below 70. If you want a definite win, buy them at below 50. Those guns are placed around 10-70. So you should buy them fast whenever they are low.
AP caps are placed around 1-40. So buy them at 30-40. Make sure you dont try to 1 cap it, chance of winning is 0.5% or you must be VERY lucky. Buy 10 of them and see if you won.
Also, when 1 million euro adder comes, don't buy any caps. Last time euro adder came and more than half of us got banned.