How can doors be broken?

So I did dome testing on this topic and I must say, I was pretty surprised by the results!
I documented (almost) all of my testing and uploaded the video to YouTube here.

TL;DR:
Doors will only break, if you shoot them fast enough, the amount of damage dealt is not that important. Any weapon, able to shoot fairly fast, will be able to destroy a door easily.
"Just spam it!"

Destruction of doors:
Doors will always have 4 stages of destruction:
(only exception are grenades)

 1. Fully intact (no holes)
 2. Half broken (top, bottom or middle)
 3. Broken (two parts missing, very few peaces of wood left)
 4. Destroyed (Door is completely gone)

In every stage, apart from the last one, the door is still functional.

Damaging:
As you can see in the first clip of my Video, I shoot the door a total of 40 times with an AR, but the door still doesn't break. Note that I shoot one bullet per ~0.85s.
Then I switch to a faster shooting pace and it only takes 3 bullets to destroy a part of the door. (Door is now in stage 2)
For the other half of the door I will also only need 3 rapidly fired bullets.

Firing Rate needed:
I did some very simple calculations on how many bullets per second must be fired in order to break a part of a door:
(These are the results of the M416)

Bullet1: 46;25
Bullet2: 47;08 (13f = 0,4333333333333333s)
Bullet3: 47;22 (14f = 0,4666666666666667s)

1f + 13f + 14f = 28f = 0,9333333333333333s

So it's safe to say, that by shooting 3 bullets within 1 second you will break a part of the door. (using an M416!)

Different Weapons:
Any weapon, able to shoot "fast enough", can destroy a door. Here's a list of weapons:

• 3 Bullets
  • M16A4 (5.56mm)
  • M416 (5.56mm)
  • Scar-H (5.56mm)
  • AKM (7.62mm)
  • SKS (7.62mm)
• 4 Bullets
  • UMP-9 (9mm)
  • P1911 (.ACP)
• 5 Bullets
  • P92 (9mm)

This list is not complete! I haven't had the opportunity yet to test all of the weapons.

PLEASE READ:
Because testing something like this is very time consuming and based on a lot of luck (finding the right weapons whilst not dying) this is all I can answer so far.
I will definitely update my answer in the future.

EDIT: By "I will definitely update my answer" I mean, that I might update my answer...