It happens sometimes when i kill somebody with the remington12gauge or the sawed-off and only if he carries the same shotgun.
If i am very close to this opponent and i kill him and run fast towards him my shotgun gets automatically reloaded in less than a second. (the HUD dislpays only something like "picked up remington12gauge" ; i can't remember the exact message , but it's as if i would pick the remington12gauge up, whithout having one before)
Maybe this sound a bit confusing, but i hope you understand what i'm talking of...