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.

You can also do this with a Stinger if you have 39/40 or 59/60 kills. Shoot a UAV with the Stinger and switch to your primary weapon before the missile hits/destroys the UAV. This will add 10000xp to your Primary weapon (or the weapon you switched to).

found by Aura


Here's a video of it in action;
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