ammo.

It all depends on what you are looking for. I used to use .4s in my rifle shooting 390-400, and now use .3s in a rifle shooting 410-412.

I would try some different weights and see what you like best. Although stay away from the .25s...too light, regardless if there is wind or not.

This might sound stupid, but is .4 the same as .40, in airsoft terms that is.

Yes.

I used .32s in a rifle shooting about 550-560 ish... Use .3s in my Tanaka at about 420 fps/.2, can crank it up to about 700/.2 though, and use the same bbs.

What brand are you all using for your BB's?

King arms platinum.
Could I use .36 bb's. My gun does 450 fps with .20s.

You can use any weight ammo that you want.

Each rifle will shoot differently, so you are going to have to play with a few and see what happens.

Madbulls and KSC's.

I would recommend .36's but dont be open to trying different types, currently madbull but I am always open to something better