With non-bullpup rifles, if you attach both a drum mag and foregrip, the drum mag will be treated as a normal size mag in both animation type and reload speed - meaning the hand clips through the drum, and it reloads faster than it should for a drum mag - quick reloads are performed with an invisible magazine.
(screenshots taken in the Frenzy gamemode)
This bug only appears to occur when you use a foregrip. Remove the foregrip but use the drum mag? It behaves normally - no clipping - normal slower reload speed.
Bullpup's don't seem to run into this issue as often as far as I can tell.
And yes, this is after the most recent hotfix. I took these screenshots less than an hour before posting.