This means you can get the Speed proficiency from Snipers/LMGs and the Damage proficiency from Shotguns on ANY gun. This also works with secondaries.
So first off you need an Overkill Pro class.
Make 2 Sniper or Shotgun classes and put either speed or damage on both of them.
Next make a class with NO proficiency at all.
Now start up the game, have the overkill class on and kill yourself and change your class to the other class you made.
Simple as that, you now have what ever proficiency the overkill class had on any gun with no proficiency.
Pretty sure I'm the first to find this, so please, give credit to "Tech" if you post this anywhere.
Thank you Techtakedown for making that video.