If the case headspaces on the case mouth you only need a taper crimp and bullet grip is controlled by case diameter.
As a example Lee makes undersized dies that reduce case diameter .002 to .003 more than a normal sizing die and increase bullet grip.
Many competitive pistol shooters use the undersize die because they use range pickup brass. The older and more fired cases become harder and spring back more after sizing and need sized smaller.
What is the diameter of your expander and the thickness of your case walls. Both will effect bullet grip and the old trick is the loaded bullet push test to see if the bullet moves after loading when pushed against the loading bench.
Your sizing die diameter could be on the plus side, the expander could be on the plus side and thin cases add to the problem.
Bottom line, bullet grip is controlled by the cases ability to hold the bullet in place and the taper crimp only streamlines the case mouth for feeding.
Below is a reloaded 9mm cartridge with a slight taper crimp. The case below the seated bullet is wasp waisted showing how much smaller in diameter case is and increased bullet grip.
Another example below and a Lyman type "M" expander .003 smaller than bullet diameter for added bullet grip. The case is bumped onto the .226 section of the expander to aid straight inline bullet seating. And after bullet seating depending on the amount of brass spring back a taper crimp can be used to "slightly" close the case mouth for feeding.
Below a Redding pistol die with a type "M" expander (center image) and again the expander diameter and the sized case diameter effect bullet grip.
NOTE, on my light to medium loads in my .357 and .44 magnums I only use a taper crimp with plated bullets, and I have no problems or bullet setback.
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