> Does one directly affect the other?

Thought experiments:

1. Bazooka-like barrel open on both ends with identical bullets shooting out each end. I would say that each bullet got approximately half the energy.

2. Fixed barrel, hard mounted, no recoil movement. Bullet should get about double the energy of (1). If you imagine the expanding gas between the bullets of (1) but then change it to operate in half the volume (split down the middle), the pressure would be double but not quite since combustion rates may differ.

3. Barrel with very long throw, very low spring force. Bullet gets most of energy, peak recoil force is low.

But always true:

  - Newton's third law: For every action, there is an equal and opposite reaction.

  - Conservation of momentum: p = m v
    considering impulses: F t = m = p / v, p = F t / m