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Stumbled upon this by accident, but I've replicated it a few times now and it does work.
What you need:
Primary weapon where you are close to earning a 10,000 XP bonus (500 kills, 250 headshots etc)
Secondary weapon (Only tested on pistols so far)
Slight of Hand Pro (May not be neccassary)
What you need to do is to get the kill with your primary weapon and immediatly switch to your sidearm, before the 10,000 XP comes up on the screen.
If you do it fast enough the XP will be added onto your secondary also. On a pistol with 0 kills I managed to get just shy of weapon level 9 by doing this glitch once. The video below I was only level 2 with my .44 magnum and the XP maxed it to level 10 and gave me a further 10,000 XP for doing so.
I failed so bad when trying this. Somehow when I was 299/300 or something with my L118 and I tried this, I got the 10000XP when switching to my deagle but it never applied to my L118. Hell, I don't even know if it even applied to my Deagle (I won't know since I was lev. 10 on it anyways). I was level 24 (needing like 1000xp for 25) and after I got my 299th kill, my bar never moved. I'm not sure why, but it pissed me off missing the 10k xp with the L118.
I tried and it worked, but I was thinking:
What if you use Overkill and Sleight of Hand Pro (for the fast weapon switch) and then do this glicht, maybe your secondary will level up a lot..?
When I did the glicht with my secondary pistol it went from lvl 1 straight to lvl 10.