Advanced Gun Scripts?

I would like to ask, how would I make a Gun script that would have a GUI, you could also do:

Grenade = G Melee = V Change Shoot Mode = F Aim = Right Click Crouch = C Lie Down = X [If it's a machine gun with Grenade Launcher at bottom]Change Scope = Z Sprint = Shift and there maybe more, but this is all I could remember... and also so that there would be:

Damage Fire Rate Bullet Drop Reload Time Speed of Bullet Accuracy of Bullet It doesn't shoot were you aim your cursor but shoots were the Barrel is pointing Less damage as the Bullet keeps going forward (best would be for system to find out how far it could go if nothing gets in it's way and were it would land and then it takes that distance and equalises Gun damage like needed) that's all I can remember from this section. thanks for Reading.

By they way, am asking this because I want to have the best guns possible for my game.