Madsen's thought on the rims catching on the cutoff area has been discussed often so I was trying to think of something else. Just in case you missed it Butlers Rangers said something about filling the cutoff lever hole with a nail filed to duplicate the end of the original cutoff then fixing that in the hole, maybe with low strength Loctite.
The other thing I thought of is with the short spitzer the cartridge is trying to enter the chamber at too steep an angle.
If so maybe adding a little hard silver solder or brazing to the inside of the side plate on the area that the rim touches as the bullet is starting into the chamber. If this is a solution, then the rear area of the build up will need to be very smooth so the rim glides over the build up.
If this idea can not possibly work let me know and I will delete the post so as not to cause confusion in the future.