Any ideas how to cure this problem? It doesn't seem to be related to any particular brand or weight cartridge.
After reading this, I pulled the tube & checked for dents, rust, etc. The follower was a bit gummy, but no problems with the tube. No burrs on the stop. Even with the plastic follower clean there is steady resistance. Not enough to make it hang, but slow it down.Have you examined the follower?
My 500 came new with some rust in the tube and the follower was sticking.
It would "trap" the shells too tight in the tube some times, and the stop would get hung.