It's the bloom, not the hitboxes in my experience. Bloom makes your bullets go sideways, behind you, above you, and occasionally disintegrate entirely. Things that increase bloom include:
- Getting shot at
- Moving
- Standing instead of being crouched or prone
- Sneezing
- Firing full auto
- Jumping
- The enemy looking at you funny
Bloom is pretty lame and I wish they'd remove it or at least tone it down. I much prefer my bullets going where my gun is aiming. If you start tap firing/ shooting in 2-3 round bursts at more than 15-20m, you'll probably start to see your shots landing.