This question already has an answer here:
I was wondering if the mAh on an battery pack is important or relevant, so you would recognize a difference?
Little bit Off-Topic:
I've read a lot about "Nerf voltage mods" where modders change the standard 3xAA NiMh batteries (1,2v each = 3,6V) against 2 Li-Ion batteries (3,7v each = 7,2v) and one "dummy" battery for better perfomance and range.
Now I'm also thinking about changing to Li-Ion (with around 1500mAh) or just buying better/stronger NiMh batteries with around 2500mAh.