Green laser pointer on my airsoft gun

i dont know much about lasers, as my name states lol, but ik that i want a green one for my airsoft gun, a custom build, i dont feel like buying a cheap one, as i stated before i just dont know much about them, i would like someone to explain clearly to me what makes the beam visible at night? i mean ik the diode does but i want a bright beam, a somewhat thick beam ik green laser beams are not really big, and one that can stay on for a long time, which mean heatsinks. what mw of laser would make the beam visible on any night no matter the conditions?

also another quick question, do heatsinks have to be around the diode or does it matter where they are, as long as they are connected to the circuit?

Depending on the conditions, 15+mW should be enough. Is this for actual play or just for target practice?

The visibility of a beam depends on the power of the laser, the conditions of the air, and the viewing angle. As the laser light shoots out of the laser, it reflects partly off of the molecules in the air, in more or less parallel direction with the beam. It will also hit water droplets (fog) and smoke to reflect more light into a viewer's eyes. Green lasers do not have thick beams.

Again, what will this be used for?

If you're using a green laser for a gunsight, at night, and it's above 5mW, you'll always see the beam, though others around you might not. You're viewing angle as you hold the gunsight will be perfect for beam visibility.

The only thing you need to figure out is how to mount the laser to your airsoft gun. If you have a rail system, I have a gun mount that can be used with it

for target practice mainly, im not really going to play airsoft at night unless its with my brothers or something. and i have red laser mounts and shit, i will probably put a green laser in an An/Peq box. but on a clear night would i be able to see it? and do heatsinks have to be around the diode or can they be anywhere in the circuit?

30mW is clearly visible at night, and should be enough for your use

it shouldn't need too much of a heatsink, but it's usually around the diode or module (vets, feel free to correct me).