Since this sounded quite plausible (well-written post by the way), I decided to test it out with some friends.
There were three of us in the game, and I decided to pick up the MP5K on Round 1 and exclusively use it for the rest of the game (not even PaP it, just in case that counted as a different weapon).
I picked up Double Tap and had the Headshot Popper (non)permanent perk, and kept them for the entire game - This acted as my baseline for damage.
By Round 18 something interesting happened! On Round 17 it took me 3 bullets to the head with the MP5K + Headshot Popper + Double Tap. On Round 18 it started taking me only 1 (ONE!) headshot to kill the zombies with the MP5K. I didn't notice any orb or hear any sound (I've never noticed a sound when receiving the permanent perks, only when losing them), but I'm convinced this is the exact permanent perk the good sir above was refering to!
We continued playing until Round 28 and I retained this perk the entire game. I did keep almost exclusively using the MP5K the whole time though so I don't know how long using another weapon would cause you to lose it again, although I would assume that you could not activate this perk on another gun without losing it on the current one.
Some observations regarding the perk:
- I started using the MP5K at round 1 and used it exclusively (apart from meleeing denizens and about 2 zombies), and I had the perk by Round 18. This might mean that you need to use a weapon exclusively for 17 rounds (matches with 17 revives for Fast Revive perk....hmmm....coincidence? I think not)
- I decided to upgrade the MP5K on Round 26, and the upgraded version still had the bonus! This leads me to believe that it still counts as the same gun when upgraded. So you could probably do 17 rounds as a combo of the regular and upgraded version of the gun to receive the perk.
- As an indicator that you have the perk (apart from the obvious damage boost), when you remove a limb from a zombie (or pop their heads off), it makes a more *crunchy* sound than normal. Kinda hard to explain this one, but the limb removal noise is definitely different. It's like bones are snapping more or something.
- The perk only applies to the gun you were using when you gained it; I also had a HAMR but it did not have the damage bonus (and didn't make the *crunchier* limb removal noise)
Finally, huge congrats to the guy above for finding this! At last, a proven 4th (non)permanent perk!
P.S. I would have analysed this further, and checked for noises/orbs etc but when I try to play my recent game in Theater it gives me "You have selected an invalid film or clip". Typical..
P.P.S Ah ha! I know exactly what it is!!! I decided to load up a new game solo to check that I still had it, which I did. I then realised that body shots weren't doing extra damage (I generally go for headshots only) and it dawned on me. This perk makes you do extra LIMB damage! This explains why headshots were doing more damage, as well as arms coming off easier, as well as the extra crunchy noise whenever limbs were being removed. So there you have it chaps, an extra limb damage perk for your chosen gun. This makes me think that it might not simply be about using the same gun for a number of rounds, but instead getting a certain number of limb removals with a gun (bearing that in mind, I had about 300 headshots by round 18 if that ends up being helpful, but you'd have to account for extra limb removals from arms and stuff too of course).
Thisis wild - today I did the saem thing , MP5 for about 20 rounds with dbltap. firing into a crownd torso level and got a green poof, then i noticed an improved damage, its was about round 12. I would agree with this theory, especially since it is tracked in the leaderboards just liek headshots, and revives. - Gibs