Hey I never get sick of seeing a solution that works. The pre-drop is something I am still playing with, but it is a change to the main body which takes days to print, so anything I come up with I want to be rock solid.
Lablover, I know you are sick of printing the same parts over and over (welcome to the club), but if you want to give it a go I just posted an even longer version of the adapter. It tapers from 7.5mm to 5mm over a 35mm distance. That is a pretty subtle taper. I could go even longer if you want.
The only other thing I can suggest is like what GWS said above. If you need to file a small rounded section right on the outside edge to make sure bullets are not leaning on that right as they fall in the hole that may help as well.