How is firing action points consumption calculated?

Yes, there are weapons which have a special effect, like "Consumes 25% less AP in V.A.T.S Mode". However, these weapons are only obtainable from legendary enemies.

In my experience, the AP cost of a weapon are determined by the following things:

• Weapon mods which start with light (less AP cost) or heavy (more AP cost)
• Fire rate (higher fire rate needs more AP)
• Sights (Scope needs more AP, while reflex needs less AP)
• Stock type (Short stock needs less AP)
• Accuracy (the more accurate a weapon is, the more AP it needs)

Against contrary believe, the weight of a weapon does NOT affect the AP usage of it.